A thin POSIX layer and ELF loader shouldn't be too much of a problem for Microsoft to implement if they wanted to do so (WinNT actually did have a POSIX personality at some point, but I don't think that's still supported). I'd also like to see a builtin WASM runtime in all operating systems.
Indeed, back in '95 or so there was a library called CrossELF that'd let you compile ELF so files and use a tiny loader linked to CrossELF for each platform you cared about to load the main .so file, and you could build platform-independent code with it. I remember writing some simple networking code where the loader just had a tiny set of shims for a few calls and the rest of the networking code was a single binary for both Linux and Win32.
The problem is wrapping the relevant APIs - as you can see w/e.g. Wine. For some functionality - like networking - the surface is pretty small, for others its a nightmare.
Microsoft did implement a not-so-thin POSIX layer and an ELF loader atop the NT kernel, it's WSL1 (Windows Subsystem for Linux). It was obsoleted by WSL2, which uses a specifically-tuned Linux VM instead for performance and completeness reasons.
I haven't played with it, but I think the classic Windows POSIX subsystem used the COFF/PE file formats instead of ELF.
