We know Gabe isn't going to sell to MS, my point is nothing is impossible. Stranger things have happened.
Anything can technically happen, but if that is all what you're arguing, then... well duh.
The chances are still extremely unlikely not just because Gabe has expressed disagreements against Microsoft in the past, but also Valve's updated policies towards game publishing on Steam are too libertarian for Microsoft's tastes. Yes, I know that the link to Microsoft's tastes is not related to game publishing, but it is to show how the fundamental principles between Valve and Microsoft highly contrast with each other.