4e practically destroyed most mechanical differences between the classes by giving everyone a selection of at-will, per-encounter, and daily powers and removed a lot of effects and interactions (especially non-combat ones) that added depth to the system, while also striving to make the system more "approachable". Many of the implementations of how classes worked became very arbitrary, though they did try to tie it all into the lore (at least in FR, what with the Spellplague and the destruction of the Weave and all).
Them going for the cataclysmic event in the FR lore and killing off characters put off quite a few people too, I'd reckon, since as of 5e they'd just retconned and went back on most changes, practically reverting most things to their pre-4e state, and returning a hint of complexity into the system, though 5e still remains very much dumbed-down and shallow compared to 3/3.5e (I still can't wrap my head around spellcasters not triggering opportunity attacks when casting spells with adjacent enemies...). Removal of skill points, BAB, and the proficiency mechanic make - at least me - feel like there's barely any progression to what some characters can do on some levels, and that there's sometimes barely any difference between a martial, pseudo-martial, and spellcasting class performing in combat. Feats being heavily reworked and made an optional feature was also a strange move from the character building perspective. From what I've read about the One D&D, they seem to actually be going back to a few of the 4e's gimmicks again (like rituals being available to everybody, for example) and reducing the complexity even further, so it might be even more of a turn-off for a nerd like myself - I guess WotC found a specific niche with the Critical Role and such, and are keen on becoming even more approachable. Meh.
So yes, Pathfinder did find its niche with many players who were rightfully upset with 4e and whom 5e didn't really impress, either - myself included. Owlcat's implementation's problem is that, while it has a ridiculous selection of subclasses and feats, there aren't too many cool tricks, interesting spells, or viable (on higher difficulties, at least) combinations of classes that are satisfying to play as. My hope for Rogue Trader lies specifically in them abandoning Pathfinder (I imagine they aren't fans of 2e either...) and going for a bespoke ruleset instead. We'll see what comes out of it.
