Flatpak nowadays feels like the spiritual successor to appimage. All the dependencies are containerized, and uninstalling an app doesn't leave behind a residue of automatically created files on your system... at least in theory. All of these benefits are kind of negated if an app has full disk read/write permission.

Appimage is kind of silly in my opinion. Appimage is just "portable application" (i.e. when an app gets shipped as a folder containing the executable, .so dependencies, and resources), but crammed into a disk image for some reason.