What I see with MS' move is that PC indie market and the console indie market. With Xbox Ones being devkits (finally)...smaller devs can get their box on PC and Xbox at the same time (yes, they won't have Xbox Live enablement unless they get those certs but still theoretically) with basically the same code (xbox one and xbox scorpion). It would leave indies to have use a lot of different code to get their game on PS4/K. Whenever (because I think it's a when and not an if) Steam accepts UWAs, developers can put their games on Steam, Windows Store, and Xbox all using the same code. If anything it leaves Sony out of the picture with indie games. Obviously that's all long term stuff, but that's what I think MS is trying to get with this.
Obviously you still have the other 3rd party apps that can be made for the Xbox and PC that aren't games (and that's where the streaming device comes in). But this whole move seems to me that Microsoft wants to get developers making UWP and making it easier for gamers to choose the Windows Store for the gaming habits. Like I said earlier, I think that Steam is going to try to support UWAs. Maybe not at first, but I think ultimately, they will. Otherwise, they'd be just like MS of old.