To quote Abraham Lincoln, "You can please some of the people all of the time, and LOOK OUT VAMPIRE!"
What I think he was trying to say, is that it's impossible to make a game (or anything else, for that matter) that 100% of the people find perfectly to their liking. Honestly, I think Mists did a good job of making most people pretty happy. In ToT, at least, hardcore players found a significant challenge. H Lei Shen has a four freaking percent completion rate, and 5.4 is live next week! Meanwhile average to bad players (such as myself) have normal, LFR, and shortly Flex Raiding to give us some measure of progress. Add scenarios, pet battles, Sunsong Ranch, and different ways to spike your reputation, and you have a lot to work with. But including all that content that the most tunnel-vision hardcore player would rather chew their arm off than play didn't ruin their fun, because they still have content that basically only they will see and work towards.
So, short answer, no. Just because there's a lot of content for casuals doesn't mean anyone else's magically vanishes.