I would say that Microsoft is not averse to open sourcing, and have opened up a few pieces of Windows already. But the Windows codebase is gigantic, containing proprietary code even from others. A file-by-file audit would be necessary before open sourcing.
Here's an example of just one framework and how long it takes before full open sourcing is possible:
Microsoft should just make Windows Desktop and Windows Server totally free. They already make money on services and apps they sell.
I've been using Linux for over 20 years and haven't looked back since.