As an outsider looking in, this appears to be Microsoft's "one app to rule them all," so it kind of makes sense. Currently, everyone wants to run Windows -- Linux, Mac, iOS, browsers -- because of the legacy of applications and games. The Windows App will allow users to use any device on any OS to access these apps by running the Azure Virtual Desktop remotely from the Windows App.
As a developer, I'm working with the Windows App SDK -- not related to the Windows App. This set of libraries replaces the previous desktop initiatives: Classic, Universal, WRL, etc. It looks like Microsoft as a whole wants to consolidate all of their Window's stuff so they can concentrate on Azure and selling services. Consumers are pretty much stuck with the legacy operating systems running browser apps. I suppose, ultimately, we're reduced to running Windows OS through the browser -- especially since the Windows "RDP" App is not working on my setup and I have to run the new Windows App through the browser on my Windows machine.
While it would be great to have a new, optimized, safe operating system, it doesn't look like anyone is motivated to do this and we'll continue to rely on the aging Linux and Windows OSes as the foundation supporting all these apps and services until someone else -- the Chinese? -- takes the initiative.