You missed a few important things.
The "universal" part doesn't matter, MS's phones and tablets are in their final dying moments so there's no need to make something that runs both on real Windows and Windows RT/Phone.
Universal apps run on the XBox One.
At this point Win32/64 programs run better and have less limitations
Unfortunately, they require porting to/from Microsoft's console. That will no longer be the case, and the XBox is setting up to be a flexible TV media device.
So it is a meaningless limitation for now. Programs using an API nobody uses won't work with Steam.
Being able to run the same code on XBox and Windows will be appealing, although AAA titles will always need access to a high-performance video API. Oh wait, UWP supports DirectX 11 and 12.
If Microsoft gets UWP working to the point where most XBox games use it, no one is going to port to Win32.
This is a doomsday prediction, and like most doomsday predictions it is based on what the predictor feels to be true, not actual evidence.
Not all doomsday predictions are equal.
End-of-the-world predictions require some powerful yet unknown phenomenon to occur.
This scenario only requires a single bad actor to successfully manipulate its own platform. And there is a history of success there.
Obviously, there can be no evidence unless internal Microsoft documents are leaked, but the scenario is credible based on known capabilities and past behavior.