You already said it. Flatpak and Snap both include an entire system around updates and rollback which provide some pretty strong guarantees for update success. AppImage does not. It's got some libs available that an individual developer could use to implement their own update mechanism but isn't a built-in. And besides, without a system-level component that manages install/update/rollback, you can't have any guarantees about the update process. You're back to the Windows-world per-app update.exe paradigm (or update.sh in Linux).