The problem goes back further than 5E. I remember being absolutely floored that Monte Cook wanted to nerf martial types further in 3E and that they were too powerful compared to casters.
I cannot for the life of me understand that line of thought. I liked 3E, even with its flaws, but holy shit, did they beat fighters with the nerf stick.
Wait really? Did he mean in refrence to earlier editions or in refrence to early 3e.
Both. Monte Cook was one of the leads on 3e, and apparently strongly pushed wizards. And for after 3e released, see his variant Player's Handbook, Arcana Unearthed.
Though I think it's less that he thought fighters were overpowered and more that he just really, really liked spellcasters.
I'm not sure the distinction matters. I understand spellcasters are cool and all (I mean, I've got no room to talk as I love to play 'em), but the more I look at 3E, the more I wonder what the hell they were thinking.
I think the psychology behind it matters, because it can help explain why it happened and how to avoid that kind of problem in the future.
Wizard supremacy has a couple of roots. One, is the players most inclined to become rules geeks and thus designers seem to overwhelmingly prefer wizards. It tickles that geek power fantasy itch, by giving them a physically weak character who can acquire REAL ULTIMATE POWER (cf. Raistlin's popularity). It also tickles the rules geek itch, because it gives them more moving parts to play with.
As a result, there's a strong inherent bias in favor of wizards. That's complicated by a couple of additional factors, one of which is the reality or lack there of of magical powers. If a designer is the type of geek who is always picking nits, what does that do to fighters? It limits them. Because they start looking at fatigue, encumbrance, whether it's realistic for Fighter Bob to walk away from a 60 foot fall, and so on. The result is a progressive nerfing of the fighter class. And since we're talking about geeks who go over every last detail again and again, this becomes multiple waves of nerfing as everything is overthought and every "realistic" restriction imaginable is put in place. And especially if fighters are defined as the anti-magic-user class or mundane class, you end up with things like 3e's feat list, where there's not a truly heroic or legendary feat to be found (until epic levels, at which point they're pretty pathetic).
On the converse side, magic is explicitly not realistic. It can't be tested against reality, and how it works varies from source to source. So instead of tending toward harsh limits a la the fighter, the tendency with wizards it to remove limits. Because you can always find an example where it doesn't work like that, so out damn restriction, out! And when it comes to powers, it's even worse, because all those different myths and legends are pillaged for the best and most powerful powers. Some of which are insanely powerful, because magic in stories tends to have plot-based limits, not some kind of internal limits. The reason wizards don't dominate the stories in every way is because they're written that way, not because Merlin is balanced against King Arthur. So you end up with a steady erosion of the natural limits built into the game system, and an endless procession of new powers that are all added on top of each other.
Any good redesign of fighters or mages needs to recognize these tendencies, and counter them.