They are making money now. It would be stupid to pull out.
Unless Natal is going to make them bleed money theres no logical reason to discontinue the xbox brand. However, I do agree that it would be stupid to go next gen without generating a early profit so that MS can continue to recoup the costs that the original XBox brand caught them. People tend to forget they lost billions on the original XBox and will make a profit on the 360. MS was in a lose-lose situation with the original XBox and thats not the case anymore with the 360.
It's just that simple.