WoW cannot be "killed" as such, it was released November 24th 2004, thats over 5 years ago! And it now has 11.5 million paying subscribers.
There are players on there who weren't born when the game was released. If WoW loses its popularity in the next year or two (or three or four) Blizzard can still say that the game had a development cycle sucess that dwarfs any other game in history, WoW will not be Killed, but rather simply Abdicate in favour of a worthy sucessor.
Games come to the end of their playable lives eventually, usually when a sequal comes out, so yes, WoW2, or whatever the next Blizzard MMO is will most likely take its place.