I’d be onboard with a Rust rewrite of Windows. Go for it! It’s the least memory-safe OS with most problems and the widest desktop deployment. Do the Rust folks do rewrites of closed source?
That’s probably in the works (on an exploratory basis at least) behind closed doors. Microsoft’s infrastructure team has noted they were switching away from C++, and the Security Response Center team considers it the best chance to move forward.
The business side puts a very high premium on backward compatibility. Until that changes, they need to keep around the cruft / shims / architectural bridges / glue that make that possible. Working with the raw Windows API is already painful enough^; I can't even imagine the effort it would take to reimplement it in Rust.