If I was making a high magic fantasy setting, instead of trying to worry about balancing casters against non casters I would just make everyone a caster and then build society around that. I consider that to be one of D&D's greatest flaws. You are making a tabletop ruleset - which is supposed to be played by people, which means it needs to be balanced - and then you are trying to tell me that someone who can manipulate time and bend reality is supposed to be balanced in comparison to someone who can't? I almost feel like its a bad joke. From my PoV, if you want to keep immersion, you either need to have a world where everyone is a caster and then a "fighter" is just a caster who uses melee spells, then you can make a balanced world, or you throw balance out of the window and tell people, "if you want to play a fighter, just know you will suck."
In the 2nd ed games I DM ed the fighters were the strongest class at the end. Especially the Paladins. At the point that the mage can stop time your fighter should have equally power items like a ring of 3 wishes and an artifact that has a number of spell like abilities.
Powerful magic items serve the same function that feats do but you aren't locked into them for the rest of your build. For me immersion comes from "adventurers are just rare birds"
I mean who really want to devote their life to crawling around in sewers hoping to find treasure in the guts of a carrion crawler?
Sure, as a DM you can do whatever you like and balance encounters that way. My issue isn't that they are unbalanced, my issue is trying to make them balanced in the first place. Just look at the real world for example, in the real world, there are very minor differences between people. Now look at the level of inequality that exists. D&D is proposing a system where the differences that exist between people are not minor, in fact, in some cases they are huge, but then its also going along and saying that the inequality which exists is not significantly worse than the real world. A system that has some people who use magic and others who do not should be, if it had any realism at all, a tyranny. There should be mage/cleric/druid/caster of choice rule, depending on which power holds sway in each region, over the entire of the forgotten realms. Thay should be the norm, not the exception. There shouldn't be a "fighter" in the first place, because in a system which allows for such power, those who have it would kill anyone who could potentially pose a threat long before they get around to actually posing a threat.
The forgotten realms is, in my opinion, far too naive about these things. In my opinion in such a universe, the only way for such an outcome to not happen is if magic is nowhere near as rare as it supposedly is in the forgotten realms - if essentially everyone has it. That way there are plenty of people who can act as a check against the power hungry, because power is essentially democratized.
