I'm glad Flatpack appears to be winning over the utterly horrible Snap, but I still don't like it. I fear a day when it becomes difficult to get software that isn't packaged in Flatpack, and I have good reason to: Ruby Gems. Long ago, I was big into Ruby, and was a major contributor (I authored one of the core standard libraries). Gems came along, and I hated them; eventually, for unrelated reasons, I stopped using Ruby altogether, and now when I encounter it, it's impossible to use anything that doesn't have Gem woven into it. Consequently, AFAIK, my current system has nothing Ruby installed on it - unless my OS package manager is doing it under the hood.

IMHO, Flatpacks are a really poor work-around for people supporting and using programming languages that don't build software correctly. Rust and Go do it right: they build stand-alone executables. Flatpack adds literally no value to software built with these. They're not the only languages that do this, but they're the ones having their moment; any language that builds stand-alone, statically linked binaries would do.

I'm with you about AppImage; it would have been a better solution. Any packaging solution requiring extra software to be installed and a service to use is a bad design. I'd be objecting less if AppImage were emerging as the winner.

Incidentally, this is why Podman is superior to Docker: yes, you still need extra software to be installed, but there's no system service with crazy, root-level permissions required to run containers with podman.