Very few former wow fans can resist a new wow expansion or at least look into it, so if they keep the games quality up, keep updating the graphics/engine I don't see how it's going to die. Blizzard still even support WC3 which has been around for even longer.
The only thing I can see that would wreck it was if they went to a f2p model in order to move players into a new game of theirs and the quality dropped due to lack of funds or something but I can't see them doing something so stupid.