"Leopard eats face" is a dumb reddit thought-terminating cliche. I invite you to actually think about the problem and ask yourself why developers of FOSS tools selling out happens in some domains but not others, and effects some users without warning but not others.
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.
Another aspect of leopard territory is API churn. On Android and to a lesser extent browser extensions, regular rebuilds are necessary to keep the application up to date. This sometimes necessitates reworking parts of the application, not just rebuilding it. This recurring chore places a constant burden on developers, they can't "finish" an application then forget about it and move on with their lives; doing so would see their work vanish. On the other hand, on GNU/Linux desktops it's perfectly feasible to "finish" an application and leave it unmaintained for 15 years, people will still be able to use it. And on Android with F-Droid, most of the burden of rebuilding applications to keep them running is taken on by F-Droid volunteers, reducing the burnout pressure on application developers.
The conclusion is simple: Strict separation of the developer and packager/distributor roles keeps the leopard away.
Oh no, only if it buildable from source. Binary compitability in Desktop Linux is non-existant, even Linus Tovarldis has ranted about this in the past. Even source is not immune to rot, if no maintainer steps up, nobody will package it, and god forbid the application uses something like QT4, while the distro decides to drop QT4 altogether...