I used to think that this would never happen, but now I wonder. MS makes lots of money from, say, office on ipad and android. MS makes tons of money from Linux via Azure. To this end, MS supports dotnet, squirrel server, and lots of other stuff on Linux really well. So maybe, just maybe, this could happen.
Yes, that playing field has shifted a lot. But Microsoft will just make more and more of their own code cross-platform (e.g. Office now exists in a modern version that's surely not very OS-specific and in a barely maintained Windows-only legacy version for anything but in name). They can sell to non-Windows customers just fine without ruining the only relevant USP Windows has, compatibility with a quarter century worth of win32 API clients.
It makes sense to port their application software to other platforms so more people can use it.
But what would they get out of giving their competitors the ability to run non-Microsoft products made for Windows? That is a big, non-trivial task which they'd be spending a lot of engineering effort on, for free, and wouldn't benefit them in any way. It would strictly reduce the number of reasons to use their products.
