Windows 7 doesn't have "many years" left. Windows 7 has less than two years left in its support lifetime. (2020 is when extended support ends.)
In the mean time, if you, in a corporate environment, must Windows 7 until the bitter end in 2020, you can use WPF as a bootstrap step towards UWP. The XAML is similar between them and a lot of the coding paradigms are same (MVVM patterns), and if you architect well (.NET Standard 2.0 libraries) you can share 100% of your business logic between a WPF application and a UWP application.
Xamarin.Forms also has an open source WPF renderer if you'd prefer to further go the cross-platform route. (Xamarin.Forms supports UWP, iOS, Android, macOS.)
I agree that I think Microsoft should have had the .NET Standard, XAML Standard, WPF Xamarin.Forms development stories in place sooner to better leverage developers that need to support older Windows, but many of those pieces are in place now if that is what corporate developers were waiting for.
In the mean time, if you, in a corporate environment, must Windows 7 until the bitter end in 2020, you can use WPF as a bootstrap step towards UWP. The XAML is similar between them and a lot of the coding paradigms are same (MVVM patterns), and if you architect well (.NET Standard 2.0 libraries) you can share 100% of your business logic between a WPF application and a UWP application.
Xamarin.Forms also has an open source WPF renderer if you'd prefer to further go the cross-platform route. (Xamarin.Forms supports UWP, iOS, Android, macOS.)
I agree that I think Microsoft should have had the .NET Standard, XAML Standard, WPF Xamarin.Forms development stories in place sooner to better leverage developers that need to support older Windows, but many of those pieces are in place now if that is what corporate developers were waiting for.