People at the end of (official support for) 3e understood many of the flaws of the game's design. They understood that the class tiers, while semi-intentional, had done pretty severe damage to the play-experience. They understood that Vancian spellcasting was incredibly overwrought and overcomplicated, while at the same time being heavily abusable by players. They understood that PrCs were busted to hell and back (hence the vehement opposition to anything PrC-like being added to 5e, because the well is so thoroughly poisoned that no one is willing to let WotC try again). People knew that high-level 3e was terrible to run, being incredibly DM-intensive without actually rewarding the DM with much freedom. People knew that the Full Attack concept had sounded cool, but ended up being an albatross around 3e's neck. Etc.
A lot of 3e's problems were known and understood. And a lot of the solutions are pretty consistent; there's a reason people who have played both games draw comparisons between 4e and PF2e.
I'd suggest that the designers' view of what was busted was different, perhaps in major ways, from the players' view of what was busted.
Like, 4e codified the Fighter as a Defender and gave it a mark mechanic. I can imagine that this was fixing a 3e problem, a problem that late 3e also tried to fix: a fighter has no inherent feature to serve as an "attention-getting" mechanic, and so can struggle with being the party tank. A problem I personally broadly agree with.
But there's consequences to that design. Like, now you've given the Fighter an explicit mechanical in-combat Role, we can't have different kinds of Fighters. We can't have self-interested mercenaries and high-damage giant-slayers. All Fighters are Defenders.
Yeah, 4e's mark is a solid Defender mechanic (not perfect, maybe, but it does its job well), but giving players that option is quite a different take than making it a core component of the class's identity. I don't think that's a fix to a problem the player base was having in a broad sense. I think it's a fix to a problem that some part of the player base was having - the part that really liked the kind of combat that 4e highlighted, for instance.
But that wasn't all of the player base! It might not've even been
most of the player base (though it might've been the most Online of the player base at the time). "The Fighter isn't a good Defender" was only a problem for some people in late 3e, and the fix in 4e solved a problem that simply didn't exist for a lot of players in a way that was pretty deeply inflexible. They had to eventually make a whole alternate Fighter class to try and fill the demand for a more hard-hitting fighter.
Unfortunately, it just really is the case that some people are attached to...frankly, busted mechanics. Being able to be the star of the show. Being the god-wizard, who elects to allow others to participate because that's more fun than just solving every problem yourself. Being the insanely powerful shapeshifter-spellcaster-pet-owner. Etc. And, on the flipside, some people are so used to slapping patch after patch after patch after patch onto their game, they no longer see that activity as reflecting that there are problems with the underlying system; they see it as just the cost of doing business.
The language of "you're attached to busted mechanics" is part of how early 4e got this reputation for a kind of inflexible arrogance.
"What do you mean you don't want your Fighters to be good Defenders, it's just an obviously better way to play!" Well, sure, buddy, but if I was playing 3e more as improv theater without a battlemat and less of a game of pushing minis around, it's not better for my purposes, is it? Pushing minis around isn't how that person plays D&D and so that's not really fixing a problem they have.
Having a powerful character who can significantly impact the narrative and game flow isn't a flaw in the design that needs to be fixed for a lot of players, so "fixing" it is removing part of the fun of the game.
It would always have been controversial--slaying sacred cows is like that. But if it had prevented the formation of Pathfinder (or, better yet, gotten Golarion as an official 3rd-party setting for 4e), straightened out its own act, presented itself as looking and feeling like a traditional D&D game even though the rules worked differently, prepared better for an incipient and extremely severe recession, and prevented the tragedies and unwise choices that repeatedly doomed the digital tools...I really do think 4e could have overcome the opposition it faced. A lack of organized resistance would have left a lot of players sticking with D&D because, as different as 4e might be, it's not AS different as something like World of Darkness or Shadowrun. It still has AC and attack rolls and six stats and modifiers and familiar classes etc.
Personally, I think some of the mechanical choices were so at odds with how a significant portion of the audience wanted to play D&D that trying to flavor them as just like the D&D you always knew and loved would lead to some pretty angry and shocked reactions. "No, my Fighters aren't like this, my Wizards aren't like this, my Elves aren't like this, stop telling me how the way I've been having fun isn't the right way."
Not that the change in flavor helped ("no, I don't necessarily want to play in the D&D designers fantasy heartbreaker homebrew, why is that the only option, why can't I play my D&D?"), but I think that the mechanics were also a big part of the stumble, because in at significant scale, they solved for problems that were not really problems for a lot of people.
