>1. The software will have one less user. (Implication was potentially many fewer given “we” would uninstall.)

And what is the injury or harm of that? The developer said they wouldn't action the change and indirectly suggested the user to fork it - "it is open source after all".

>2. The software will have one more competing product (forked from their own work).

This is the very nature of open-source software. Labeling it a "competing product" makes many assumptions that aren't earned. We're talking about a user saying an opt-out feature should be opt-in. If I forked LibreOffice because it contained features I did not like, am I now making a competing product? Am I harming or injuring the repo I forked? Am I fighting the LibreOffice developers?

>Most people willing to put their time and energy into building software for others would consider losing users to be a moderate negative and having their work potentially forked and having to compete with a derivative a strong negative.

The developer didn't seem to consider it a moderate negative.

>“We will fork” is a strong threat. It is basically a statement that “we” will fight the maintainer for the effective future of the project.

You're expanding the scope of what was said. Saying "I will fork your open source software" [to make an opt-out feature, opt-in] is not "fighting" the maintainer for the "future of the project" - it's a core element of open-source software.