Even if everyone agreed on Apt as the standard package format, wouldn’t you still need to create multiple packages for the various different versions of libraries each distro will still have depending on their release cycle? As far as I know, it can be done theoretically, but since libraries can often break ABI, it’s safer to bundle all dependencies, but then you’re not far off from an appimage in practice.

Also, what are your thoughts on Richard Brown’s (of opensuse) talk on Flatpak, who was a prominent hater of containerized apps.