"WinRT" XAML runs great on Windows 8+ and is mostly forward compatible with UWP XAML (especially 8.1+ universal XAML). (Though statistically "everybody" that was on 8 is now on 10. It's 7 that will be the next XP.)
Windows 7 is the outlier, but this isn't that much of a different argument from the complaints that Windows XP had terrible out-of-the-box support for WPF. The same arguments stopping people from using UWP today seem very deja vu to the arguments against WPF even back when in Windows 7.
UWP XAML isn't even that different from WPF XAML. I've got a feeling for some LOB applications maintaining a UWP and WPF build side-by-side is relatively straightforward (and definitely more straightforward than those of us that for various reasons worked on side-by-side XAML builds for WPF and Silverlight).
Windows 7 is the outlier, but this isn't that much of a different argument from the complaints that Windows XP had terrible out-of-the-box support for WPF. The same arguments stopping people from using UWP today seem very deja vu to the arguments against WPF even back when in Windows 7.
UWP XAML isn't even that different from WPF XAML. I've got a feeling for some LOB applications maintaining a UWP and WPF build side-by-side is relatively straightforward (and definitely more straightforward than those of us that for various reasons worked on side-by-side XAML builds for WPF and Silverlight).