UWP is a platform that makes it easier to port and make games on PC and Xbox one with little to no coding. It still has issues like all new things. So if we think about it, Microsoft increase their install base significantly and became more relevant than ever. That is my two cents at least. Also, Microsoft stated in one of their leak documents supposedly that they believe anything that is running windows 10 is an xbox as well that is why you're about to get the xbox one interface on PC soon. 

UWP is dead on arrival as far as PC game developers are concerned

Care to explain? It has issues right now, but it is new and did help make games like Quantum Break and soon Sunset Overdrive on the PC without much effort.

UWP?? Don't you mean GFWL2
The only time anyone is going to use that is when Halo 6 drops. It's like the WiiU of PC except it's exclusives are even less enticing.

Developers aren't going to do release their games on Windows store and not steam. Valve and 99% of PC developers from indie to AAA will stick to steam, ea will stick to origin, Ubisoft will stick to uplay, cdproject will stick to gog and steam... etc.

Other than microsoft and possibly mobile developers next to no one will be using uwp.

People hate it, and devs have to make backwards-compatible version anyway for Windows 7 and 8. No UWP for Win 7 or 8. UWP is bad for anything not exclusive to Windows 10.

Not sure how much you know about the PC gaming scene but GFWL2 will never be accepted by them. MS already screwed over PC gamers once before. There are pretty much two type of PC gamers ones who use Steam and ones who use anything that wont require a DRM like Steam. Neither of these two groups are going to move to MS. They'll buy the few games that they have to but that's about it.

Maybe they hope it increases its install base so much that they will be able to drop the actual console completely?

It's all well and good microsoft claiming any windows 10 pc is an Xbox, but microsoft would much rather you have real Xbox...

Microsoft don't get money when you buy pc hardware, microsoft don't get a cut of every piece of software (the 99% that isn't sold on Windows store) unlike Xbox.

Microsoft have less control over the pc market and how you use your pc compared to Xbox.

Microsoft wishes you'd use an Xbox for gaming not a pc, simple as. Gaming becoming bigger on pc isn't a good thing for microsoft, no matter if it's running on their os. If windows store was the main way to buy games on pc only then would they want PC gaming to succeed, but it's not and it never will be.

Less xbox's sold and more PC's sold is a bad thing for microsoft.

The only reason microsoft is pushing all their software onto windows store is because Xbox isn't performing as well as they'd like so they're trying to create a locked platform they have control of and get substantial revenue from on pc, unfortunately for microsoft this isn't going to happen.

There you have it, girls. Console war is over. X1 is already leading by a few hundred million consoles. Looks like Sony really dropped the ball on this gen.