UWP is built on top of a substantial portion of Win32. Removing Win32 would kill UWP as well as many first-party applications which are written directly against Win32. But it would also kill server development, driver development, third party runtimes, and a host of other use cases. UWP would need to work for all of those, and developers will need to start utilizing UWP for those use cases, before Microsoft could start thinking of killing off Win32.
I think we would see a new operating system, built from the ground up around UWP, well before we see an NT derivative without Win32. (I would personally love to see a UWP layer on a Microsoft-branded flavor of Linux, though I don't know how feasible this really is.)