Microsoft used to have and develop XENIX a long way back, before transferring it to SCO. https://en.wikipedia.org/wiki/Xenix.
Opinion disclaimer, please disregard if you disagree.
I think it might be the best strategy going forward for MS to swap the NT kernel for linux or a BSD-based one. Think of all the tooling that would become instantly compatible. IMHO, the closed-source NT kernel went from a strategic asset to a byzantine liability in the past 15 years; as the quality and breath of the open source alternatives have grown.
Closed-source Windows can live on powered by a different kernel, ala OS X.
Think of all the software that would become instantly broken. There needs to be a long and slow transition for this to happen. There are decades of software that is compiled against the Win32 API (or wrappers around the Win32 API).