Microsoft has every incentive to want to close the Windows platform, from general control of the platform, to stronger DRM enforcement, to security, to getting a revenue cut of every app that's ever going to be built for the Windows platform (including games).
The only reason they can't close the platform all at once is because they know the backlash would be huge. But on the bright side for them, they still have a huge monopoly in the desktop OS market. They've already gotten quite gutsy with all the privacy invasions and forced updates for Windows 10, and it doesn't look like the backlash bothers them all that much, as they've continued to push through with all of those things.
So they might try to push this one step at a time, like deprecating the open desktop platform every year, until most developers are forced to just choose the closed platform.