It was never really about the "over-simplification" as someone who was there through the age of Welfare Epics I can tell you that is untrue.
It also stands to reason that while Cata was a bad expansion, MoP by comparison was not and in fact it was incredibly well received.
It's what came after that killed WoW:
1. Extreme oversimplification. There's oversimplification (Welfare epics, cutting bloat) and there's what WoW did, they cut far too hard far too fast and left all the classes feeling like a hollow shell of their former selves, Devs finally admitted they overcooked it and started putting some things back.
2. Endless boring grind as a requirement to participate in endgame content. This right here, this has been an issue that recurs every expansion, it has pretty much killed the majority of the raiding scene over there with the exception of those that absolutely no life it.
3. Rehashing the same story beats. Garrosh was the last interesting villain. They have no idea how to make a compelling villain despite having a whole team of writers.
4. Stubborn refusal to admit they got it wrong. See YoshiPs quick statement about Job PvP balance after they got it wrong compared to Ions takes several months of doubling down before finally admitting it's wrong. There's an argument that the XIV devs are absolutely getting it wrong with housing and I'm absolutely on the side that they are getting that wrong with a few other irritants (overdrawn lockout phases, repeated content dropping too late in a patch cycle, etc) but they aren't in game system issues that affect every little thing you do, like they were in WoW
5. Being rude toward the paying customer. I can't name one instance where YoshiP has ever been rude toward the subscriber base, I can name dozens where Ion and the other WoW devs have been outright hostile toward theirs. They fostered a very abusive relationship toward their customers and that was always going to end badly for them.
6. Catering to a loud minority of the playerbase. Ion was right about one thing, they were making changes and putting in systems that people wanted. What he failed to say is it was for a very small minority of hardcore raiders that did not care what impact those changes had on the majority playerbase which at the time were Midcore casual. Yoshida does make changes.....but for the larger demographic of the playerbase, the hardcore raiders might not enjoy those changes, but they are the loud minority and we've already seen what changing a game on their whims does, so would rather not go down that road again.
7. Public exposure of toxic workplace culture. We all know this had an impact and will continue to do so for years to come, this was the straw that broke the camels back for many and most will never touch another Blizzard product again.
I could keep going, but that covers most I think.