You're making a tremendous assumption to claim that "casuals" were content to just kill the same bosses over and over again. Maybe, after doing that for so long and getting nowhere, they got tired of it, or felt that, after having experienced raids in WotLK, the whole "killing the same dungeon bosses for no reason" suddenly wasn't going to cut it.
Except you could very easily have killed the NORMAL max level dungeon bosses... yeah, again, that didn't cut it. The exact same activities were available; to claim that suddenly people couldn't just "mindlessly kill things for 'fun'" is completely untrue. That's untrue now.I mean it's fine for you to say it cost them 15 bucks a month but yea that's the point. For casual players killing an easy boss is fine because well they're casual. They don't want to put up with to much bs either because it's difficult or because it's time consuming (which has the same net effect if your bad at killing bosses). Properly understood the complaint from cataclysm was not so much that bosses were hard it was more that my time is being wasted here and I'm getting nothing or very little out of this experience.
It just gets old after a while, among the myriad of factors you're also ignoring.