> And QT, a popular window toolkit, is quite expensive unless you release your software under GPL
'Doc, it hurts unless I do this.'
Then do that! Don't be a software hoarder.
> Also, a single maintainer does not have time to work on three or more window toolkits for their app (Windows Forms, Cocoa, and GTK).
As I mentioned elsethread, X11 & POSIX are supported on Windows, macOS, Linux & the BSDs. Just write against that, and you're done.