It would be very useful that Xamarin.Forms support WPF (Windows 7) for the following reasons:
1) There would be a single framework to develop all kinds of applications, mobile and desktop.
2) Manufacturers of third-party controls could focus their efforts on developing controls for a single framework.
3) Windows 10 is not having the expected degree of implementation, there is a very high percentage of computers running Windows 7 yet.
4) Enterprise applications are not usually distributed by a store, so the UWP option is not as suitable as WPF.
5) For many desktop software development companies moving to develop their mobile solutions in Xamarin.Forms would be a very easy step.
Please consider this option, we are many development companies that we consider this would be a great success.