Agreed to your point about combats that end quickly favor ATK over DEF, but complete disagreement to your point that DEF lets you tank big hits while HP lets you tank many hits. I believe it's actually the other way around in an additive formula (and neutral in a multiplicative formula).
Because DEF is a stat that provides value on every hit you take, whereas HP is a value that is raised once and provides no extra value until you get a (free) full heal, e.g. at a save point or an Inn, the value of DEF against a single hit needs to be lower than the value that HP brings (as a whole).
For example, in an RPG where the formula is a.atk - b.def and all numbers are relatively small, you'll usually see DEF values raised by like 2 on a level-up, whereas HP will be raised by 10 or more. Sometimes the difference is even more extreme and it's like 100 HP or 2 DEF (this is usually where stats' magnitude is multiplied via formulas, such as the Epic Battle Fantasy series where you have stats like 7 ATK, 5 DEF, and 2,000 HP).
Putting this into the perspective of dungeon run vs. boss battle, it means that HP is actually the stat that you want to build in order to survive huge hits from a boss, since (using your own formula) 10 HP will help you take 10 more damage from a huge single hit without dying, whereas 2 DEF will help you take just 2 more damage from a huge single hit. Over the course of a dungeon where you're sustaining, say, 30 hits on your way through, though the 2 DEF will decrease your total damage taken by 60, which is a lot better than adding a measly 10 HP to your total.
Long story short: HP for surviving huge hits, DEF for tanking many hits over time (in additive formulas). I believe this holds true under any circumstances (except wildly imbalanced stat systems) and any factors, though I'd be open to counterexamples.
For multiplicative formulas, since the DEF stat is essentially multiplying your "Effective HP", I believe that ideally you want to sort of raise both in equal proportions in order to most effectively survive either big hits or repeated hits.
The reason I think that way is because of the way most of these RPG's are designed. You'll get into roughly 25-35 encounters in a single dungeon and one shot most of the enemies in those encounters. You'll have a few where you take some damage (especially if you're under leveled), but the damage is already so minimal that those hits really don't matter to most players (players will only heal up from the dungeon damage at certain thresholds where they think they'll need the extra HP). So, the amount of HP typically just matters for the dungeon crawl itself. How many hits can the player sustain before needing to cast "Heal" on someone. I don't deny that DEF plays into this, but in general, that's the way HP seems to work. It's a "fuel meter" to get you through the dungeon. Essentially a resource that needs to be managed. The fewer hits you take, the less "refilling" of that meter you need to do. You can accomplish this in many ways. Overwhelming attack... formulas that allow you to zero out damage via a defensive stat... skills that nullify/reduce damage... raising evasion... lowering enemy hit rate... there are a lot of methods by which to extend your HP, but in general, it's use is tied to "tanking many hits". But, DEF on the other hand... if the stat itself never comes into play (due to ending battles quickly, as we've discussed), then its purpose is to tank those massive hits that Bosses do. Most RPG design follows these two rules. HP tanks all the really tiny hits from the regular enemies in a dungeon. It's rarely necessary to refill this during a dungeon until close to the end... or you rarely refill it more than once. However, if the boss has an AOE or a Damage Spike attack... The DEF stat comes into play to keep everyone alive during that combat. The higher your DEF in that instance, the less those Damage Spikes will hurt you. In essence, the DEF stat isn't "tanking many hits" for you. It's tanking a few hits that are necessary to tank to survive.
For example, you have four party members, you trudge through the 35 encounter dungeon... you've sustained roughly 300 HP damage (in this instance, we'll give everyone 800 HP) across all four members equally (just for the sake of the example) so everyone is only missing 75 HP. Your defense came into play on each of those hits, but you've sustained only maybe half a dozen to a dozen hits during the whole dungeon run. Limiting how much damage you took wasn't near as important as those enemies not hitting a "threshold" to cast Heal magic... or drop a consumable. Their HP let them tank the many hits. But, then they heal up just before the boss (most players do this, just to have the best chance of winning, even if they're a few HP short... not sure why, but it's interesting

). They step into the boss room... and that boss has a single attack that does a lot of damage. The party doesn't have to tank "many hits" now. They have to tank a few hits that will do a lot of damage. If that boss uses his big attack every few turns; every third turn, lets say (I'm using this just because bosses usually have 3 or 4 attacks, and we're just saying they use everything evenly and randomly for the sake of argument), then the party has to contend with this large hit every few turns and it will result in needing a heal after each hit... unless the player has high defense. That high Defense will tank the powerful hits to prevent the player from hitting the "threshold" of needing to heal and run their resources dry quickly.
I'm sorry if that doesn't make a lot of sense. Basically, I'm using the concept of "when does the player need to act/react" for how these stats work. In practice, they won't need to act/react much at all in the dungeon because their HP won't reach the "threshold" for requiring action. Thus, the HP is being used to tank the many hits they'll sustain along the way. But, in a boss fight where spikes of damage can occur, they will be "acting/reacting" more frequently and healing away that damage far more frequently. That is, unless they've got higher defense to tank the large hits that require the frequent heals.
Does that make any more sense?
Formulas that de-emphasize defense in such situations make those heals even more frequent in boss fights than they might otherwise be (like the default ones for the RPG Makers). It's generally why I prefer the trade off to those that use simple addition and subtraction. My Defense isn't de-emphasized and a player can use it to heal up less often during a boss fight, but the trade off is that sometimes they will do zero damage to an enemy or the enemy will do zero damage to them.
I genuinely love how we're focusing in on the intricacies of comparing stat allocation to football team management

(I wonder what the Cleveland Browns' stat allocation looks like?

)
There's probably something to be said about the effects of grinding alongside an all-out attack strategy, and comparing it to me
(a lanky 6'0", 160-pounds-soaking-wet nerd with skilled hands and great gamesmanship, but average athleticism) taking on the NFL's dumbest running back, and how he'd still crush me (both in terms of winning a football game, and literally) no matter how much I outsmarted him on the field. At some point we probably have to be okay with letting players brute-force their way through obstacles if we're allowing them to grind.
I don't mind letting players grind if they want to. Or even Brute Force things. If a player chooses to spend a little bit of extra time to gain some extra stats for a perceived edge, I have no problem with that. I just de-emphasize that edge in boss encounters where stats count for less than they do in normal encounters (because the bosses use gimmicks instead of just tanking a ton of hits and dealing a lot of damage). I also removed all stat gain from levels too... so you're getting indirect power from Levels now instead of Direct Power. I ended up putting the stats behind equipment (where the largest boosts will come from, thus making balancing a little easier on me as a dev) and doing Quests (as incentive to do as many Quests as possible rather than a ton of combat to then breeze through a Quest).
Personally, I just prefer if all stats are useful for something in a game. As a player, I find it a little annoying that I can equip a piece of armor that has 50 defense on it... but it only results in like 20 damage being reduced from normal hits. Because now, at that point, I'm thinking, "the game is lying to me about how effective this is". I just personally dislike the idea of a stat point that is useless.
But the larger point I was making was that, unless you're grinding to extreme levels and getting some stats automatically on level-up, an all-out attack strategy where you're dumping all of your points/equips/whatever into ATK or MAG is unlikely to work against bosses, where it really counts. Even if you could kill a boss in 3 turns (unlikely), if that boss can one-shot your party with his AoE, it's not going to matter. Sure, it's nice to wipe every normal encounter with a single skill; sure, it's nice to be able to grind more quickly by doing so; sure, that's 95% or 98% of battles... but that glass cannon approach won't cut it against bosses in any turn-based game, so the player is going to have to invest in some DEF if they want to progress in the game. (With that said, games should certainly do a better job lining up the things that you have to do, with the things that feel good!)
Yep, I agree. But, many of those games have a default amount of DEF they give you at each level and it's usually enough to get you through boss encounters by using an all-encompassing "brute force approach" against everything. My larger point is that for those 2% of instances when defense matters... you're investing a lot of resources into that. You buy the equipment, you maybe distribute stats into Defense, just for those 2% of encounters. In fact, you're spending the vast majority of your money for the defense there. That 2% can also be negated by simply overleveling. In Final Fantasy V, I personally had this issue. The minute the game gave me jobs and a ship, I went looking for the easiest way to get JP so I could max out as many jobs as possible, so I could swap them in and out as necessary or use their buffs that were useful. The side effect of this? I maxed out the first set of crystals fairly early into the game... and I was level 38 when I did so. I proceeded to steamroll the next 10 bosses in the game in one to three hits. Even with the low level equipment. Your example hinges on the boss hitting your party with a powerful AOE. But, if not all bosses will do that, then the player can still "brute force" the boss, not worry about defense, and just use "Revives". This is a tactic I use in Pokémon against the Elite Four when they're powerful. Gain more levels? Grind? Switch Pokémon? Nope, I saved all the money I got in the game, spent it on nothing except Pokeballs I might need... and when I got to the Elite Four, I dumped it all into Revives as they're cheap, can be used every single turn if necessary, and keep me in the fight long enough to get hits in or to run the enemy dry of their most powerful skills.
One of those examples is one in which I wasn't trying to break the game, but did so as a side effect of doing something else I wanted to do... The other example is one in which I broke the game by not grinding and by simply dumping all my resources into just coming back from the dead.
In the case where the player's ability to allocate stats is less (such as a system where stats are given automatically for level-ups, and the player only uses equips and/or small bonus point allocations to adjust stats), yeah, then the all-out attack strategy can be fine, and it probably feels more appealing to the player. But even then, from a raw mathematical standpoint, the stats seem to be more balanced against each other than you believe they are (as discussed a lot above, as well as in
@Aoi Ninami 's post).
I think it largely depends on the combat system being used. What I've found through most of my gameplay of RPG's is that once you step outside of the "predicted realm" of where the dev thinks you should be for any given area... the stat importance of everything other than "Attack" drops off the map. You don't need Defense because you kill everything in one hit (or close enough to it, that it doesn't matter). You don't need MP, 'cause you're doing very little healing or even spending it on using Magical Nukes. You only need a single point of speed more than any enemy in order to get first turn against them (unless turn order is that of active time battles). You don't need Magic Defense either, 'cause you're one-shotting things.
It largely begs the question of, "What if a player knew they could nullify all damage from the baseline enemies with Defense instead of relying on raising their attack to end the battle quickly? Would they do it that way?" I mean, we used to have cheats for "God Mode", which simply made you immortal and immune to damage... which were very popular. Often more popular than cheats that let you kill everything in one hit... So, it begs the question. Which is a better design perspective? Are they equally valid? Should it be an option to nullify 100% of damage through Defense in the same way you can nullify 100% of damage through attacking and killing an enemy with a single hit?