Activision has so much money, but at some point the WoW trend will slow and they will start losing subscribers. However, even if they lost half of their subscribers overnight they'd continue to make money for years to come. Shoot, people still play the first Everquest game, or Ultima Online.
After a few months of world at war, people noticed that it wasnt as good as call of duty 4. So it already did overtake world at war multiple times. The only chance of people continuing to play world at war is if the zombie maps get really really good.
Anything requiring a gaming-standard PC is ultimately going to get eclipsed by a box costing a few hundred dollars/pounds/balls of lint. For all of PC gaming's advantages it's still a black hole to throw cash into, hence why so many just get on with gaming consoles instead of worrying about the MHz/GHz rating of the memory in their latest graphics card (hope selling that kidney was worth it). Cripes, give me a console any day.