I think it's probably unlikely not because of any opinions of Microsoft, but because Windows is vast and contains large volumes of code they've licensed to use in closed source code, but not bought the ability to re-licence as open source.
Yes, that's also the reason Raymond Chen gave when people asked Microsoft to open source the Pinball game that was removed from Windows Vista: https://blogs.msdn.microsoft.com/oldnewthing/20121218-00/?p=...
In the comments, a reader traced the chain of ownership and apparently Electronic Arts now owns the Pinball source code: https://blogs.msdn.microsoft.com/oldnewthing/20121218-00/?p=...
source: https://blogs.msdn.microsoft.com/oldnewthing/20180515-00/?p=...
If it was released without the third-party code, then it'd be a research project, which'd be pretty neat but not something most probably expect from "Windows is open source".
But I don't doubt there is other code that is not so easily replaced.