I wouldn't discount Starcraft 2 I think its sales will be huge. However Starcraft is so ingrained in Korean gaming culture it might take a while before the pro players transfer over to it. It wont be an overnight transition like some games have. It will be a gradual shift that will take years most likely.
Back on topic. WOW is appraoching 10 million subscribers, I seriously doubt that Blizzard will release a sequel for a very long time. I believe the second expansion will get the userbase over 10 million and that its growth will continue albiet more slowly for years to come. Even when the game begins its decline I doubt they'll quickly create a sequel. They'll more likely leverage the hardcore fanbase with some more expansions and try and maintain its profitability.
