> but it was too complex to maintain and Microsoft decided to take the easy road.

The insurmountable problem was file system semantics. In Linux, files are dumb and fast. On Windows, files are smarter and therefore also slower. Linux applications expect dumb fast files and they produce a lot of them. Mapping Linux file system calls to Windows file system calls is doable but you can't overcome that difference.

At that point, simply virtualizing the Linux kernel itself is an obvious better solution long term.