I think it's perfectly acceptable for project maintainers to provide rpms, debs, ppas, etc. as long as those artifacts are organized in largely the same way that distro maintainers would organize them. (I doubt any distro maintainers would disagree.) I don't think it's a good idea to pack lots of already-distro-packaged libraries into those artifacts. Building for many distro formats is a pain, but I doubt that building this new format in a universal way will be easy either. If you have a different AppImage for each distro/version combo, is that an improvement?

For the packages you've mentioned, live active projects with maintainers who can produce this distribution format but can't deal with apt, maybe the giant ball of wax is okay. I won't be installing anything this way, but clearly I'm not the target user. It seems certain, however, that in future we'll be hearing about lots of vulnerabilities attributed to AppImages linking in old library versions.