It's not that easy.
I agree that supporting Windows helps its ecosystem.
But also open source software on Windows is an important gateway to the free world. When you are already used to Firefox, LibreOffice and VLC, you might as well switch to Linux painlessly, but if those didn't run on Windows, switching to Linux would require relearning everything.
Irrelevant. If it's time to stop using windows, all those windows users will have to relearn everything either way. Whether they do it in a windows environment or a linux one doesn't really change the equation.
A sudden lack of software on windows will increase user migration. If we all keep publishing for windows, users will just stay there because their needs are already met.