Since it had been repeatedly mentioned here. I'm going into a little controversial route to wholeheartedly disagree about difficulty slider / change difficulty mid game, story mode / normal mode represent a lazy developer balancing their game. As a rule of thumb, no game ever balance in the first place. You can't balance a game to serve them for both casual gamers and hardcore gamers.
The change mode mid game is not really a bad idea. There're many types of players. One that challenge themself (try to beat the game without legendary item, lowest level, etc. And even not changing the difficulty mid game), one that don't want the trouble and want to enjoy the content (changing difficulty on the spot they particularly don't like), one that make a mod to cheat everything. Nonetheless, they're all enjoy the game the way they want. And I think it's wonderful.
It was mentioned that "I'm going to change difficulty when I'm gonna buy stuff so it's cheaper" things are 'meta gamers' where this type of gamers always min-maxing game so they gain more with lesser loss. Of course, there're some people who complain about the game can be exploited that way, and you have a right to complain if it's an online game. However in offline game, are you really going to do that? of course some of you do, but do you really "enjoy" doing that? if not, why u do it?
As a disclaimer, I'm not sure if it's not derailed from the original topic. I'm just throwing my cents here.