I'm guessing you mean something other than FOSS (free / open source software) in your critique of FOSS. Reason being, Swift is FOSS (https://github.com/apple/swift), both the compiler and the standard library. Likewise, the Microsoft C# compiler is also FOSS (https://github.com/dotnet/roslyn).

You seem to enjoy closed-source platform libraries (WinForms, AppKit), and want to promote developing software that relies on them. The survival characteristics of this approach don't seem very strong. Good luck!