And the counter-counterargument is that it grew then not because it was difficult, but in spite of it. The growth curve was consistent with a model where vast tracts of MMO virgins were attracted to WoW as their first MMO, but were not retained by it.
At best, one might argue that the perceived difficulty might have attracted people, who then realized they weren't as good as they imagined they were (truly, a "you think you do but you don't" situation). In that case, keeping the game hard would not have retained them either. The players would also have been immunized against the false attraction of difficult content, so other MMOs that tried that would have failed to get traction (hello WildStar, which had weak initial sales in addition to very poor retention.)
In this model, the MMO genre is declining because it is inherently self-destructive. It grew fine as long as it had new customers to pull in that weren't experienced with its central contradiction, but once those were used up it had eaten itself alive.