I wasn't thinking of Desktop although I would love to see something strong emerge there (other than Avalon).
Ah, curious what made you feel that. We ported massive code bases of ASP.NET and commandline tooling over to .NET Core since the 1 and had not many issues. It's a much better experience now but it never felt unfinished to me.
I have to admit that I have been writing software for a long time and one of the things I automatically do is abstract (not too far, just far enough) the underlying implementation of whatever I make/made. So our old ASP.NET code was very easy to port for that reason; I never use internals directly and still do not. For instance MVC looks more or less the same anywhere so I just use plain old C# classes as controllers so they can be reused by apps, other (non ASP.NET) frameworks, commandline, tests etc. It adds a thin layer below them so they work but it saves a lot of time and with the coming of .NET Core it proved smart once again.
Maybe you are talking WPF / Desktop only; there are other options for Mono but yes, there you would be right. However the trend is, unfortunately, toward browser interfaces (Electron etc) and those you can do on Linux/Mac already with .NET Core.