Considering there's really no real information about the game other than that it's in development, I doubt we'll see a release for at least 5 or 6 years. Who knows where WoW will be in that time.

Also, if WoW drops to say, 6 million players by then and the new MMO takes half the playerbase and gains the same amount back, that's still 3m in WoW + 6m in the new MMO for 9m total. I'm sure many of those 3m will keep active WoW subscriptions, so it's win-win for Blizzard.