I think there's a lot of "I don't understand this, but Sweeney says so, so it must be right" in this thread. Or appeal to authority, if you wish.
There's also a lot of hyperbole "Microsoft wants to kill PC gaming", "MS are evil" and what have we. What it really boils down to is, that Microsoft has created a new format, that everyone can use if they wish. They have also created a new shop, in which they want to promote said format. That's it. You aren't forced to use it. You can sideload apps written in UWP, or you can use the shop, whichever you want to.
Apparantly Sweeney thinks that in the near future, Microsoft will kill off/deprecate Win32 support, and along with it kill off Steam, Origin, Uplay etc. In the instant that happens, hundreds of millions of gamers and professionals will never give a dime to Microsoft again.
In which world is that a realistic scenario? You do realize, that the most widely used graphics software (Adobes CC software) and Autodesk doesn't run on UWP and probably won't for a very long time, if ever, right? You do also realize, that 99% of all existing games for PC are based on Win32, right?
This is fear-mongering, just like when Gabe Newell opened up about Windows 8. His predictions didn't come true, and neither will Sweeneys.