Here's the thing. All these companies want to make money. Companies like Microsoft and Sony actually make more money off game sales. Consoles are sometimes even sold at a loss.
People are assuming that Microsoft is going to make all these games exclusive in an effort to sell more Xbox systems. They might do that, but I'm not sure that would even be in their best interest honestly. Activision makes a lot of money selling multiplatform games. Microsoft would really only lower Activision's value by making all those games exclusive. Sure, some of the hardcore fans will buy Xbox systems so they can play the games on there, but I'm not sure that would offset the cost of losing all the potential customers who choose to stick with the PS.
In the end, I think Microsoft would be better off keeping all the games multiplat, and just giving perks to Xbox users. On another note, I honestly think moves like this are less about winning the console war, and more about Microsoft setting itself up to be a game development powerhouse. They might decide they want to be the biggest game developer out there and leave the hardware to others.