If MS could use their inside knowlage to create a working stable version of a windows kernel that could run on a nix platform,
But doing so would admit that *NIX is a viable platform, and better than the current microsoft one at that or they wouldn't even consider switching to it.
MS acknowledging that Linux/unix isn't just a toy would be the most damning anti-windows statement they could possibly make. Keep in mind: Microsoft is a marketing company first, and a technology company second. Even if a *NIX-based port of Windows were the best thing for them to do from a technological point of view, essentially switching would be the same as admitting defeat -- by a toy, of all things.
It'll never happen.