WoW is just old. The playerbase changed, which is only natural. It's not about casuals or pros or whatever people play the game... we are all human and we just can't take playing a game for such a long time. The gamplay is the same old thing it was the last 8, almost 9 now, years.
-Its graphics barely advanced (nope, not even the new player models will change this fact, if they not bring a WHOLE set of new animations... like casters with a staff actually holding that thing in their hands while casting etc.)
-the fight mechanics never really changed (this is probably the worst thing of all. Because autoattack+ability spam isn't as fun in 2013 as it was in 2004.)
-the storytelling is the only thing that got better over the time.
People may have instincts similar to animals, and may be psychological influenced like them, but only a small portion of them can be persuaded to play the same game for such a long period.
I don't say WoW is bad, it's just past it's prime and time. It was a phenomenon and should be handled with the respect it deserves. Maybe that's the reason the Devs are too shy to use their manpower and budget to take new directions. But if nothing changes it will slowly, very slowly come near death, but being ruined and die? Hardly. Before that it will be F2P and at least a couple of million school kids will have their turn with it over the next decade, trust that. WoW may very well endure into the late 2020s or even 2030s, we won't know how fast hardware will advance. People are still playing Pong, Pac-Man and Tetris. Maybe you will sit in the train in 20 years, bored, and switch on your internet connected brain chip for a quick heroic dungeon with your mental interface <,<
WoW did hit the world, it hit it harder than most games, maybe even all of them. It won't die for decades and its idea and story will last even longer. The same as Super Mario and others will be there in decades too.