Some of the largest projects on F-Droid are all funded either by having a paid version on Google Play, accepting donations, or a company funding their development by hiring the maintainers. Most of the software you can install from a distro repository are also developed in large part by people that are compensated in some form, if the project requires continued development.

Money always ends up in the equation when you have to invest years of your life maintaining and supporting a project. And that's perfectly fine and healthy, because it means you can find a path that does not result in either side being exploited.

As for your last point, I was not responding to developers selling out their users, which is unfortunate, but to the expectation to not have a premium/paid version of a software project distributed. I think the developer even shared the pro version for free on F-Droid (OsmAnd does the same), yet the existence of a paid version was regarded as something negative by the user above.

If you're uncomfortable with the taught of you or at least someone else funding the development of the software you use, you're only setting yourself up to be exploited.