You mean the tons of people that opened their web browser, typed thotbot/maxdps/elitistjerks.com and looked up what was the best talent set up for every spec? Yeah that was so much work. /s
The sub drop came from a combination of things. PvP was an outright mess through most of Cataclysm. The fact several classes were changed for the worse didn't help. On top of that, endgame, AKA the thing that Wrath had introduced a LOT of people to, was a lot harder to get into and progression/gearing up took longer because of how content was tuned (which in turn led to insular player behavior which meant a lot of people hit a progression wall). LFR and the catch-ups introduced with the final patch of the expansion helped a bit, but that didn't do much due to Dragon Soul being lackluster. Only then we can include the folks that did not like the talent changes leading into MoP.
As for the change to the talent system itself, most people who complained didn't understand why the change was made. The talent system was a load of false choices and had been like that since the first change made to it leading into TBC. And some wanted their false choices solely for the illusion of progression over a system that offered slightly more variety but with less pushes of a button. The complaints were no different than if people complained that instead of getting served mud sandwiches like they had been for 4 years, their sandwiches now have ham and lettuce and actually edible things in them. Sure, WoW's devs eventually took that system in a stupid direction (entirely killing the consequence from picking talents by making talents easily swapable), but the intent behind the change was solid.
This assumes everyone loves the changes and job growth going into Heavensward, which is not the case. Sure, some have taken to the new systems, while others simply adjusted and tough it out.They are called expansions for a reason, and if your character that you've spent hundreds of hours on, is made lesser not through any triumph of evil, but because the powers that be have declared a play style that you've come to cherish as "too complicated" for people who aren't going to dodge that AoE no matter how simple the game is for them, no amount of shiny baubles is going to fix that feeling of "used to be able to do that cool thing, but now you can't because reasons"