You can't cross-compile for "free" when you are switching to an entirely different architecture. It doesn't matter if the game is downloadable or on a disc.
Yes, there is some cost there somewhere, but it would certainly be easier than backwards compatibility for disc based games. The majority of Xbox Arcade games are already running on Windows, it would not be an incredible feat to bring them back to the X1.
The point is that they're not going to, and they don't care, which is something that was an ok answer back in the days when games were always on a cartridge or a disc and intrinsically tied to the hardware, but once you've got a digital distribution system up and running, killing backwards compatibility is a real crap policy compared to what Steam offers, what Sony appears to be offering, and even compared to what the Windows Store seems to offer.