I actually think this could happen one day under Nadella. May be not whole OS but part of Windows.
And the FAT filesystem driver: https://github.com/microsoft/Windows-driver-samples/tree/mai...
And also PowerShell: https://github.com/PowerShell/PowerShell
And most of .NET: https://github.com/dotnet
Maybe they could consider an Apple-style approach: open source the core of the kernel and text-mode user space but leave the GUI closed.
Of course, open sourcing everything would be even better, but that might too big of a step for them. Open sourcing the non-GUI core could be a good initial step, whether or not it ends up going further.
Calculator (the horrible UWP one, unfortunately): https://github.com/microsoft/calculator
As for that FAT driver, it was traditional to include in the SDK/DDK sources of some of the actual drivers in Windows, so that one is not a surprise.
I think that is what could happen in 10-20 year period. As long as they could drive enough revenue from Cloud.
At least M$ under Nadella so far has a much better strategic play than most other tech CEO.