Right for me there is no game that will 'kill' WoW, you just can't kill something that popular. It's still increasing in popularity.
Trying to to kill it with another game is ridiculous, they will only accumulate a fan base from somewhere else and possibly take a few fans from WoW. But WoW's current strength would require something on the epic scale of Avatar to derail it.
But WoW will eventually disintegrate on its own. Every game is maintained by something new each time you play it unless you love the game's concept so much you can continue regardless. But for a number of people WoW is maintained by the updates, expansions and patches.
But a game concept can only last so long before it gets boring, by adding patches and expansions you're just slowly trying to polish a diamond or add ever increasing extras to make the diamond look better. Now you may say that other games have lasted for eons by just doing the same thing. These games were not MMORPGS that you have to pay for.
The necessity to pay to play, means to make the money worthwhile you have to spend a greater amount of time playing the game and thus the repetitive and samy nature(I do not refer to the repetitive grind of playing, but to the actual way in which this is done) begins to become ever more obvious.
You cannot play WoW for an infinite amount of time unless you are an obsessive(true obsessive, 20 characters level 80, across 3 different accounts obsessive). The experience starts to repeat, thought you could ignore it to begin with it becomes ever more obvious. As if you had an infinite life no matter what you experienced, you would experience it in the same manner your personality would dictate and everything would descend into monotony. You need an end to make the experience worthwhile.
However, Blizzard may have the genius ability to add totally new concepts to WoW and keep people interested. This is perfectly possible as you can add new players, but without the endless patches and expansions the repetition of style becomes more obvious.
But you never know Blizzard are very good at this, I respect them tremendously, WoW is a fantastic game, brilliantly made and superbly continued. I also respect anyone who can get £8 a month off people for basically the same game.
But I think eventually WoW will be overtaken, or I hope so, some variety would be nice, and I haven't spoken to my friend since he started, I'm beginning to wonder if he actually leaves the seat I always see him in.
Oh well sorry it's so long.