Is this really the biggest pain point? I'm not sure this is a dire need for most businesses. WPF has a compatibility layer that allows developers to use it with .NET Core on Windows. Xamarin.macOS exists if you want the native look for macOS. There's already GTK bindings if you need something cross-platform, and a variety of other OSS projects (Avalonia and Eto come to mind, but I know there are others).

I'm just not sure x-plat desktop is a place where MS needed to step in to fill the void.