I think Activision knew this would probably happen. However, they did the math and decided that not paying the allegedly promised bonuses would a better deal for them financially. They decided that they still own the Call of Duty and Modern Warfare IP (including the names, story, characters, and most importantly the code and much of if not all of the game engine itself) and they have Treyarch who showed that they can do a great job with that engine and most likely do not demand the same kind of bonuses and pay as the heads of Infinity Ward.