I keep on saying this...the killer of World of Warcraft is....World of Warcraft.
See, the thing is, while we are saying that X game or Y game will kill WoW, ultimately, the problem is that the game is completely out of ideas, and cannot expand on their universe without the jaws of life inserting more story into it. Heck, they couldn't even think of a protagonist to attack with this newest Pandaren expansion.
The thing is, World of Warcraft is too big. People have scrutinized every move, and it has come down to that World of Warcraft is killing itself. It is committing sepoku on itself by releasing more and more content. Meanwhile, they are racing to develop Titan. They have said multiple times that it isn't WoW2, but you know that, eventually, they will have to turn it into WoW2 to keep the profits from the game from spilling out.
The bleeding is already starting to show. While 1million players isn't much in the size of World of Warcraft, it is showing a shift of power out of WoW and into other games.