That's a common misconception. WotLK was actually the moment WoW stopped growing and plateaued. The customer peak was reached DURING CATACLYSM PRE-PATCH. Vanilla and TBC saw huge growth, that WotLK stopped dead in its track.
What WotLK did was to bring a ton of new players, BUT to drive away a ton of veterans, both balancing pretty much equally. There is a reason the expression "wrath's babies" exists and only applies to this single expansion. WotLK was when the "new WoW" killed the "old WoW" and paved the way for the decline of the game. Cataclysm made promise to go back to a more meaningful game, but it crashed down when Blizzard only made hard 5-men that were easily outgeared, without changing anything else.
In fact, for all the talk about "overtuned heroics drove people out", the fact is that even more people left after their nerf than before. It tends to indicate that the outcry was overblown and what made people leave was deeper than that.