Agreed; there's a natural lifetime to games. WoW is rolling through its middle age now and its health is starting to get a bit "off".
However, WoW simply is more playable than its competitors. I played a few of them, and WoW was still better. The next MMO to carefully build itself to be super playable will be quite a hit, I think. Note - when WoW was released, mobile wasn't here. If most casual gamers have moved to Facebook/mobile instead and won't get a desktop to play their MMO, the market will have shifted such that there won't be another WoW.
However, WoW simply is more playable than its competitors. I played a few of them, and WoW was still better. The next MMO to carefully build itself to be super playable will be quite a hit, I think. Note - when WoW was released, mobile wasn't here. If most casual gamers have moved to Facebook/mobile instead and won't get a desktop to play their MMO, the market will have shifted such that there won't be another WoW.