That's what I was getting at -- the defense that package managers handle everything in the existing system falls apart as soon as you start compiling packages from source, adding custom repos, or installing proprietary packages, many of which will statically link their dependencies or use custom versions of libraries. There's nothing a distro maintainer can do about them.

The "authors won't keep their own software up to date" point isn't an argument against Flatpak in specific, it's an argument against custom package sources in general.

If we really don't trust authors to update their own software, then we ought to be pushing for even stricter sandboxing and system isolation than what Flatpak currently provides. The cat is already out of the bag: people already run software on their system from the AUR, from private repos, from custom Linux installers that just bundle everything into a tar.gzip. Dynamic linking won't save you from them.