When you watch Mahomes play.....I don't think there's any way the Bears could have ruined that guy. It's clearly not just scheme and coaching, the guy processes information lightning fast, has insanely good accuracy, feel in the pocket, and does stupid stuff like no-look passes that are right on the money. I think he's easily a HOF player and would have made Matt Nagy look like an offensive genius.
I think when it comes down to QBs, you can break them up into three categories: Guys who were never gonna make it, Guys who with proper coaching could be above average to very good, and Dudes who were always gonna be Dudes. I think that middle category is likely smaller than we'd like to think. Mitch Trubisky, I think, is in that first category. No amount of amazing coaching is going to change the fact Mitch doesn't have that special talent to be a starter in the NFL. Buuuuuut, it's possible Fields is in that middle category of guys where coaching matters. It's kind of one of those things you can usually only tell after the guys career is pretty much over already.