Every delay doesn't cost money and sales. Blizzard has a huge player base. And like it or not, fans game hop. Most true gamers can play the strategy games when they come out, but when the next RPG game comes, they get that and play that. Blizzard releases Ghost a FPS, people are going to hop to that. You arn't killing oyur userbase when you have 6 million WOW subscribers double that of the next best MMORPG.
If anyhting, Ghost is released and it sucked ass, you think it would not be more devistating in the long run. Blizzard has never released a bomb. They never released a C&C Renegade type game that disapoints. So rather than release a game that they feel isn't good enough and keep a reputation for the best games ever, than release Ghost and sell a few thousand copies, but have people pissed at how shit the game is.