Earlier quoted context omitted.
I don't know what you are talking about. Microsoft built a Linux compatible subsystem for Windows and also built (well, acquired) .NET VMs for OSX/Linux. It's trying hard to keep people on the desktop - any desktop.
You're kidding, right? WSL is not related to cross-platform GUIs we're talking about, and .NET Core doesn't support Windows Forms/WPF - and probably never will, for the reasons mentioned above.
It does actually although you are correct in your general point.