Oh, I think it is extremely likely that, assuming no major leadership changes, Microsoft will open parts of Windows. But I think that they'd follow the macOS model - open the kernel and parts of the low-level user land. Enough to make small embedded systems in Windows, make developers happy with low-level profiling capabilities, and receiving more external contributions.

I don't think they will open source things Windows in a constellation where people could start making Windows distributions (which could lead to large incompatibilities) or run GUI apps on Linux or Windows.