What I'm responding to is "if one day Microsoft decides to phase out... right now the only official way..." If they ever decide to phase it out, it won't matter

What I'm responding to is "if one day Microsoft decides to phase out... right now the only official way..." If they ever decide to phase it out, it won't matter what support UWP had mid-2016, it will matter what support it has when the phase out happens. Also, it's officially sanctioned right now to sideload downloaded programs not installed from the Microsoft store. Microsoft has publicly promised never to close the platform, and they aren't so powerful that they can afford to go against their development community like that. There's nothing at all indicating that Microsoft is going to shut out competing app stores and sideloading. The only evidence is that some people feel that Microsoft wants to do that.