This actually has tradition, see the Irix and POSIX environment subsystems of earlier WinNTs. I think the current approach by Microsoft owes to the developments in the OS and virtualization tech space both in hardware + software over the past 20 years. Otherwise, if Linux were equally dominant in the dev/server space, we could've seen a revivial of the subsystem feature. Which would have been pretty cool, from an implementation POV, in its own way.