IF WPF was still the primary UI tech on Windows I probably could get behind implementing such a library.
Anyone that knows their way around WPF, can make use of XAML, MVVM and all related UI concepts, regardless of the API differences between WPF, Silverlight, UWP and Xamarin.Forms.
For me they are all the same.
Main differences is that XAML processing is implemented in C# in .NET, and in C++/CX on UWP.
XAML on UWP supports static binding at compile time, and on WPF it is only dynamic (x:Bind vs Binding).
Finally the actual classes backing the UI components aren't 100% compatible, but do have most of the same methods and properties.
Anyone that knows WPF, shouldn't have any issue with XAML/UWP.
Most stock UWP components (the ones that come with the OS) are implemented in pure C++ using WRL (https://docs.microsoft.com/en-us/cpp/windows/windows-runtime...). This is mainly because they were already being actively implemented long before C++/CX became usable.