I honestly find your post a bit confusing because you're comparing running the editor to running the game...they're two entirely different pieces of software. It's like saying "If my computer can run Firefox, it can run Photoshop"
In a vacuum I'd agree with you, but in practice I've found that barring extenuating circumstances (namely, the three things Andar pointed out), the engine has always seemed to take more to run well on a given computer than a game made in that engine. Why? I could not tell you, it's not like it makes sense to me XD
I would expect anyone who is using Steam
You know what? Slightly off-topic, but I would expect anyone using Steam to have the requirements to run Steam (and I know people who don't). My computer can barely run the battlenet launcher, so I had just assumed it also could barely run any Blizzard game (which I was wrong about, Starcraft 2 runs mostly smoothly).
We all do need to be mindful, but assuming we've done our due diligence... that's where this topic is coming from.
I totally get the assumption for purposes of the topic, but I would like to bring attention to it. If we really are doing our due diligence when it comes to programming, our games will run
vastly better. You can really see this when you have day one titles on a console running far better than a title released five years later for the same console.
Of course, you also face increasing odds that modern computers will experience unexpected problems trying to run things that old properly.
I recently found a treasure trove of old rm2k/3 games I played back in the day (as in '99-'05) and wow has this been an issue.