No filesystem access for a flatpak app just means it cant read host system files on its own, without user permission. You can still give it files or directories of files through the file explorer for the app to work with, just that it's much safer since it can only otherwise view files in its sandbox.
[...] aren't there some folks who want flatpak/snap/appimage to basically replace traditional package managers?
There might be people who think that, but that isn't realistic. Flatpak is a package manager for user facing apps, mostly gui apps.
The core system apps will still be installed by a system package manager. I.e rpm-ostree on immutable Fedora or transactional-update/zypper on OpenSUSE MicroOS.
Snap can do system apps and user facing apps and fully snap-based Ubuntu might come in the future.
But this won't force people to use them. Traditional package managers will keep existing for system apps and maintainers will proabably keep their gui packages in the repos.
Never did. In fact, you interpreted my comment as if I disliked sandboxing - I like it. But whenever I defended it, people would go nuts about how that's "against the philosophy of Linux" and how Android is an abomination and so on.
But for future reference, your "argument" was terrible - "nobody says that" and "says the person speaking for the whole community" are the kind of thing that might, possibly, work on kindergarten.
There's Obfuscate, an image redactor, and Metadata Cleaner which is self-descriptive. Both works properly without any filesystem access at all, because they use the file picker portal to ask the user for the files to be processed.
I remember in 1995-ish or something when I used the internet for the first time using the Netscape browser.... And I was asking a friend if he had tried all the web sites yet. Just got a weird look back.... :) I didn't know what the internet was back then at first.