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.
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.
No, that's the thing; they ideally would only need to replace the OS. Many long years ago, when I switched from Windows to Ubuntu (this was back when it was good), part of why it was so easy is because I mostly kept the same applications. If you use eg. Firefox, VLC, open/libreoffice, audacity, etc., then you can install a new OS, reinstall the same applications, and barely have to change anything. That's huge.