Wishbone: No, it is designed to make them
run on desktops, tablets, consoles, phones, etc. "Work well" is an entirely different matter, and UWP does
not guarantee it.
They guarantee that your app works well on all of these by adding code to your app to scale it and change it's face depending on screen size, input method, etc. It's all stated in their docs on how it works.
Wishbone: As an end user, why would I care whether the app I'm using on a given platform runs off the same executable as on another?
Many reasons. Maybe you want explicitly one program to deal with your files. After all, you can download OfficeSuite on Android as you please (or whatever Microsoft Office knock-off that is being sold on the Windows Store for Windows Phones), but you'll find the compatibility is less than ideal for a Word doc you were editing on the native Microsoft Word for PC. Having one program to deal with that issue sure is a life saver, over finding an app designed for the environment it's working on, with good quality and compatibility.
Wishbone: You can already have all of this without UWP. You are saying that UWP will make things easier for developers from a purely technical perspective. This is true. I am saying it will not automatically make things better for end users, and will make it easier to make it worse for them. This is also true.
At the developers' expenses of course. Last I heard of such a cross-platform saving system, it was in Butterscotch Shenanigans' Crashlands, and they force you to use their own servers to do the cloud-saving bit PLUS whatever DRM they threw in on your platform of choice. Of course to get on their own servers, you need a Butterscotch ID. Microsoft makes it more lovable for developers and us end users by providing the framework already built, needing only your Microsoft account do that cross-saving/file sharing and whatnot. Same app to download on both devices, my same files to access immediately. I get to benefit from this.
Wishbone: As is everything else you've mentioned.
And Microsoft plans to bend over to these developers with the aforementioned tools. I say this should be the farthest they go in hunting out those lazy developers. They've given them the ability to make the same app across multiple devices with multiple interfaces, complete with the same compatibility and whatnot.
That said, it's not like your Universal App is forced to work on all platforms. No phone can run Quantum Break now, for no phone yet has 16GBs of built-in RAM or whatever. And some apps aren't designed to work on PCs either. It's all up to the developers to utilize these tools very well, and when they do, the end result is going to be pleasant.