> Or better yet, just switch everything to full POSIX.

Really not possible as most of POSIX semantics arise naturally from the kernel (or are enforced/executed at the kernel level). Windows technically provides some of them (or semantic equivalents) so you could make something work, but in order to do a full port you'd need to strip out too many concepts for it to be worthwhile. For instance the idea that "everything is a file" or the single root filesystem layout (which iirc is segmented deeply at the kernel level).