I agree that Microsoft closing down Windows in a single step would be untenable, as we’d see an open revolt by users, developers, and publishers.
But, imagine a gradual series of forced-updates to Windows that each add incremental new OS features that only work with Windows Store or Xbox Live, and incrementally add friction to win32 apps and software from outside the Windows Store. The UWP “side-loading” option, which defaulted to “off” when Windows 10 shipped, was the one example of this friction, as was Windows 8’s limiting of some of its most-marketed new features to the closed API. Over the course of dozens of updates over several years, Windows could be closed down through a series of carrot-and-stick maneuvers, finally reaching a point where only developers and power-users would regularly obtain software from independent sources.
This is the real scenario I worry about, as no single forced-updated would be so onerous as to cause a revolt, but the cumulative effect would be to close Windows down without complaint. This is why Epic is taking a position on UWP now. If we head down this path without clear and specific technical commitments to openness by Microsoft, it will be impossible to back out, lacking some sort of dramatic regulatory intervention.