That would free up hundreds (thousands?) of MSFT programmers to concentrate purely on the 'Windows' UX while leaving the underlying OS to be maintained by the open-source guys.
Microsoft could fairly easily support most of the existing Windows applications on top of almost any other OS, if they want to support capability based security, its mostly a matter of denying all file system access by default, and restricting it to only resources set up at install time, or via a PowerBox from the user.
Desktop GUI programs just have to switch from calling a dialog, then using the result of that dialog to then open a file, and instead just take the handle from the PowerBox.
I once worked for an OS company. It is difficult to replace the incumbent commercial OS with new OS because of the ecosystem around it.
I once suggested internally at the company for a different graphics stack for the OS and the next day got quite interesting reply from a very top-level tech executive at the company :)
Lot of people at the companies like Microsoft, Intel etc know they have to change, but it is like moving hundreds of hundred pound gorillas at once. So either they don't do it or it takes lot of time to change. Microsoft has been, by the industry standard, very successful at managing change.