Microsoft will do this. They just have to go a little more slowly than Google or Apple because there's such a long history and expectation of being able to run any apps. But they're gradually working there way there just like Google and Apple.

Starts with scary warnings for unsigned apps (with a workaround), then they start imposing extra restrictions for unsigned apps, and then they make the SmartScreen workaround more difficult to enable (maybe it needs a registry edit), then they'll remove that workaround in certain markets/editions (maybe the Home version first). Finally they'll remove it everywhere.