What you seem to envision is Microsoft strictly becoming a Linux distro and throwing away all their external display management code, for example. I don't think that's ever going to happen. I don't think Microsoft will ever join an existing user interface; they'll bring over their own.
What seems to be happening is that Microsoft is laying the groundwork, ever so slowly, to sit their user interface on top of Linux, without sacrificing any backward compatibility or loss of functionality.
What seems to be happening is that Microsoft is laying the groundwork, ever so slowly, to sit their user interface on top of Linux, without sacrificing any backward compatibility or loss of functionality.