The WoW audience consists of multiple complex parts. At the very bottom you have people who just play WoW and no other games, be they MMOs or otherwise. They might quit, but they won't be picking up a new MMO. Then you have the normal players who play a little bit of this and a little bit of that, possibly multiple games at the same time, you might lure them away, but it'll take quite a bit of critical mass. At the very top, you'll have the crazy people who will flock to any new MMO, complain that it isn't WoW and go back to complaining about WoW a few months later. There's a good chance they're subscribed to a bunch of different MMOs, none of which they actually play.