You mentioned RedHat was behind flatpaks and we all know Canonical is behind snaps. Who's the "big boy" behind AppImage? I'm not going to tell anyone they specifically need one of these package formats, but one of the reasons I favor Debian based distributions is that .deb is more commonly available than anything else when I try to download software not in a repository. So .deb is receiving the defacto benefit of Canonical's weight—it may not be what Ubuntu wants to support anymore, but it became the package of choice for many developers and companies because of Ubuntu's outsized adoption.
I see two ways to avoid Canonical, RedHat, or another third party dictating their packaging: either make everything available as a .deb, AND a .rpm AND .whateverarchuses AND AND AND, which is non-feasible, or have a community supported, distribution independent package. The alternative is that large numbers WILL adopt snap/flatpak/whatever for convenience. If I were a new user running something not Debian or Fedora based and wanted software outside my repos, I'd be pretty screwed if not for the existence of distribution independent packaging. Even Valve, which maintains its own Arch distribution for Steam Deck, only provides .deb.
I prefer native packages for the reason you stated: simple package maintenance in line with the intention for GnuLinux. I avoid snaps to avoid Canonical's control of them. My ideological preferences start to hit turbulence when I can't get software I want to use, though. I see two ways to avoid Canonical, RedHat, or another third party dictating their packaging: either make everything available as a .deb, AND a .rpm AND .whateverarchuses AND AND AND, which is non-feasible, or have a community supported, distribution independent package. The alternative is that large numbers WILL adopt snap/flatpak/whatever for convenience. If I were a new user running something not Debian or Fedora based and wanted software outside my repos, I'd be pretty screwed if not for the existence of distribution independent packaging. Even Valve, which maintains its own Arch distribution for Steam Deck, only provides .deb. (AUR is a thing for Arch, but I don't really like the idea of random people repackaging source for easy download.)
Even those developers who DO provide .debs sometimes do it "wrong." Off the top of my head, Discord has no repository. Instead, the client itself does a version check and if it needs updating, has you download a new .deb the way you might download a new .msi on Windows. Their platform independent alternative is a tarball that includes dependencies. Setting aside any possible vulnerabilities that need addressing (not because those are unimportant, but because they can be addressed and aren't inherent to flatpaks), this seems worse than a flatpak, because the user just gets a mess of files, no version maintenance, no reliable application location, no easy uninstall in the same place as everything else, no attempt at security.
Flatpak doesn't SEEM to be directly handled by RedHat the same way Canonical lords over snaps, and I think it was you who pointed me to the OBS flatpak when the native was working poorly, because having the dependencies it was built for packaged with it made a difference. So there is a clear, if seldom visible, benefit to packaging this way, too.
So, if not snap, flatpak, or AppImage, what then? Because as I've illustrated above, there is value and in some cases need. If those in the community with clout reject all distribution independent packaging, the result isn't the "big boys" packing it in and saying, "gee, I guess everyone just wants native packages." It's abdication of a voice in the discussion. There are and will continue to be platform independent packages. The best thing those who care about keeping Linux out of corporate hands can do is come up with their own that complies with their ideals, the same way the original package formats were derived. The next best thing is to be very clear about the least of the three evils.
Though, to come back around to the start of this: who's behind AppImage, and do you dislike it for any reason other than being divorced from system updates?
Edit: Oh my god I need to learn to be more concise.