Flatpak does need support for terminal and daemon applications such as those running on a server in order to make it a viable alternative to Ubuntu's snaps. I think it can be done, but it's a bit more difficult to run a terminal application that is installed via flatpak. I mean who wants to type out flatpak run org.gnu.ls' just as an example? Or how aboutflatpak run org.apache/httpd'? And would such beasts even work correctly? This is where snap gets it at least a little right, even though it's really clunky at the back end. Having said all this, I think the appImage concept makes the most sense of all this. I can just download what I want and make it executable, and I can even symlink it into /usr/local/bin if I want. The appImage is self-contained, and it doesn't need to be installed at all. This works for server/terminal applications as well as desktop applications. The only thing somewhat lacking is a central repository, although such a thing does exist; it's just not all that comprehensive right now. Still, the fact that I can make an appImage available on my own project website is just so damn cool! I will say though that Ubuntu would in fact appear to be shooting themselves in the foot by disallowing options users want. It's supposed to be all about choice, and if I want flatpak, I think including it and enabling it by default is the better option, as it allows me, the end user, to choose the experience I want. After all, I can always turn it off or even uninstall the package if I don't want it.

from my point of experience you talk about different things. AppImage is also quite cool but have the disadvantage of a missing store.
snap is OK for professionals or servers. But I prefer on server Podman, doe to the fact it is poor open source and can use Docker container as well.
But for standard users, either podman, docker and snap are overengineered. There you need a more lightwight system. The best are the default packages of the distro and on second the more common standard of many distros Flatpack.
Conclusion: In our LUG we have much more trouble with *buntu due to perforamce issues on old PCs (e.g. firefox via snap is not able to run web meetings with WebRTC but a non snap firefox or other browser with webRTC does it on the same hardware).
Conclusion: I wouldn't propose *buntu based systems for beginners. Better is to use a Debian flavor or e.g. Tuxedo-OS-2, which is based take over many good things from KDE Neon but w/o snap support and included flatpack with great Discover Software manager which handle it perfect transparent.

@Ulf I actually wouldn't recommend Debian for desktops at all, and I definitely wouldn't recommend it for beginners. Debian suffers terribly from stale packages, which people running servers seem to like, so it's more an LTS type of OS for server admins. Debian, even unstable, also tends to mix versions of components of desktop environments. I ran a Debian stable system for a time, and needed an upgrade to the latest GNOME for #a11y purposes. Looking at the packages, I saw two different versions of the components of the GNOME desktop, even in the stable branch. Upgrading to unstable gave me three versions of the components. Even experimental didn't help.

I'm actually more likely to recommend Fedora to new users, as it keeps everything grouped together by its major versions, has an easy to use installer, has flatpak enabled and working by default (flathub support coming to 38), has snap and podman available, has a predictable release lifecycle, and only withholds updates if they would break something. These days I use the MATE desktop, a11y and I just like the interface, and the only version mix I see is point releases of a couple of things where some packages see point releases upstream and others do not. Major versions are always in sync. Following GNOME developments, I see the same thing here. I have been running Fedora since 35, have upgraded quite smoothly all the way to 37, and haven't had any major problems. Maybe it lacks the polish of Elementary or Mint with their downstream modifications that make things look better and maybe more beginner-friendly, but Fedora is definitely good for both new users and oldheads like me, and it seems to work more closely with upstream developers, even though it is in actuality more of a corporate distro like Ubuntu. I especially like how it withholds nothing from us, giving us its own package management system, Flatpak, Snap and Podman. The only thing I find missing are the appImage tools that build appImages and the appImage launcher that automatically registers appImages on the system, but I'm hoping these things can be added at a later date.

Regarding appImage not having a central repository, there is AppImageHub, but the best part is its decentralized nature, where I can have a project website, offer my appImage for download right on the website, and I can even supply a checksum that can be verified so that anyone who wants to ensure they have my release and not some kind of compromised malware can verify it. As Flathub is essentially becoming its own corporate walled garden of a sort, complete with financial subscriptions, the beauty of appImage is in its decentralization, especially since no appImage ever has to be installed. They just work(tm).

I have no experience with Podman, so I can't make an argument for or against it, except that I did try playing with Docker containers and found them quite clunky for running server applications. For example, why should I run 5 Docker containers to get 5 websites working? That uses more RAM and more disk space than just setting up virtual hosts for the 5 sites. Then again, reverse proxies work with or without containers, but the Docker images are just so much larger and are harder to get running. AppImages are in fact larger than the system packages, but it seems running so many Docker containers is even larger. Still, if Podman makes this easier and less clunky, then I would certainly be willing to give it a shot. But I can more easily reverse proxy to an appImage than I can get a Docker container working at this point. Perhaps it's just me, as I like to take the easy route wherever possible.