Meh, I can't see WotLK stealing many people, since it mainly just appeals to endgame players, not new players. It will be interesting to see where the story goes, though.
Sc2? Well, people are STILL playing SC, even after the superior-in-every-way WC3 came out, so who knows, maybe it will be the same for WC3, the core of devotees hang around while everyone switches over the to the bigger and better. It's like going on Battle.Net on ROC. There are still a few people determined to use it these days, even though the majority have switched the superior-in-every-way TFT.