losing integration because “containerized”
Bollocks. I've seen that many times with Flatpak (can't speak for Snap), and every single time it was either because the packager failed to set up permissions or because the user messed with permissions that the application needed. Break off the tip of a screwdriver and it will no longer function as a screwdriver.
And I know you're talking out of your ass because AppImage isn't even sandboxed.
taking GBs of space
That part is true and accurate, and for a very good reason: dependency pinning. System packages can break if they don't have the correct versions of shared libraries. If a package requires a very old version of a library, and doesn't link it statically or supply it with the package, it can misbehave, have missing features, or refuse to even start. Flatpak (and probably Snap too, can't speak for it) solves that by letting the packager specify (pin) the exact version of a dependency. If five separate packages require five different versions of the GNOME application framework, then they will download five separate packages of the correct version. AppImage solves it by being monolithic: everything is packaged together into a single executable.