I could see them leaving, as others have said it would be bad because of the lack of competition, but the ways Microsoft spends money it does not seem like they are in it for the long-haul. It's like they are getting in the pool slowly, instead of just dunking their head.
My main reason is because of the limited in-house development. If they were truely committed for the long-run, I think we would have seen them to invest in building first party studios as oppose to getting timed-exclusivity on GTA DLC. If they want to leave and go back to what they had before, then not being tied to game studios would make it a lot easier.
Also, Halo is not getting any younger, and Call of Duty has a stronger hold on the FPS crowd; I feel like Halo's future as a massive console selling IP will be more limited than it has been before. Not to say it won't move a lot of consoles, it will but to the extent it has already? Quenstionable.
Moreover third party exclusives are going the way of the dinosaur. While Gears of War is a hit, it's almost done, and there is no indication that Epic will continue to make AAA exclusives for the Xbox. In the future, what third party exclusives, will either be finished or multiplat. Microsoft needs to create more critically acclaimed/commericially successful exclusives. Which goes back to my first point, limited first party support. Unless Masterchief becomes as timeless as Mario, the future of Microsoft first party support is foggy.