Why? UWP is the future of application development on Windows. That's like saying Cocoa will never be a success. Also if you can make UWP apps, you can make Xamarin apps which are cross platform (for mobile at least).
https://blog.xamarin.com/preview-bringing-macos-to-xamarin-f...
That's kind of the point. Most apps don't need unfettered system access and many (most?) app authors aren't capable of writing an app that's secure anyway; one of the biggest attack vectors today is through apps.