They would need to support customers who need that compatibility. So either they start being major contributors to WINE or something like it ... But then what about people who need a propriatary driver? Maybe a VM with hardware passthrough....
But they already have WSL for compatibility in the other direction. Not good enough?
It won't happen all at once. Linux already coexists with Windows through WSL and virtualization. The real work is in porting the APIs that the core MS products make heavy use of (DirectX, .NET, GDI, etc.).
The old Windows kernel will run in a hypervisor as needed. Windows already takes advantage of virtualization for everyday programs for security purposes. It wouldn't be that much of a shift to virtualize the kernel too.
More and more of the MS stack is being ported to Linux, seemingly in preparation for this. As of recently you can run PowerShell on Linux and run .NET "Core" on Linux. Since Visual Studio and Office are all .NET, if the Windows graphics/GDI layer is also ported to Linux, then it should be possible soon to build Microsoft Word for Linux.
Why would Microsoft want to adopt a less technology interesting kernel stuck in the age of monotliths?
WSL is there to sell Windows laptops to those that would rather buy Macs without any interest in Apple's eco-system, rather using them as pretty UNIX for GNU/Linux work.
They realised the mistake of not giving first class support to the POSIX subsystem and nowadays Linux compatibility is more relevant than POSIX.