It doesn't make any sense. Microsoft doing that will kill the only advantage they've ever had in the OS space. And they have tried it once and it failed miserably. It won't happen and it won't work. People would just stay on older versions anyways.
Besides, UWP is just a native application platform. It is not the store. I'd rather they build it with a cross platform native app framework but well, a native app seems like a tough ask nowadays.