MS is fine. They can sit on their 1st party laurels, making Natal party games, all they like.
Halo saves the day, if you've forgotten. Saved the XBox 1, gave rise to the popularity of the 360... Reach will keep the 360 going for another 3 years, long enough for another Halo installment to happen.
The first non-Bungie Halo... *that's* where it gets ugly. MS shouldn't sweat until then.