I suspect the general sentiment at MS is that this may be the last time they ship physical hardware, or at least there will be a LONG gap before the next time they do.
It would be extremely hard to try and implement or change a licensing model say 5 or more years from now, so they wanted to be forwards looking to a time when downloads are the primary way people get games.
But I'm guessing.
I wouldn't be surprised if there's an Xbox Two three years from now with full backwards compatibility and no BluRay drive.