DISCLAIMER: I usually install most of the stuff from the package repository of my distro (Opensuse Tumbleweed, Manjaro ARM, Debian, Raspbian).
But...
We could perhaps make all the flatpak that use the same libraries, like share them. You know, to reduce package bloat, disk footprint and RAM requirement. We could call it "shared libraries" for example.
Jokes aside... that's very close to what Docker and Flatpak are doing. Docker works in a system of layers. (Most of the dockers people use would most likely be extending a ubuntu:latest base)
And Flatpaks are built atop of "runtime" (base system).
These app containers only differ in the main application running and its specific collection of dependencies which are not part of the base system.
are all the same "performances be damned" approach to solving dependency hell
They are not VMs. They are not entirely separate whole-system installs.
They are closer to provide a single specific set of common dependencies that an application can target.
You need to make sure that you application works successfully on top of the latest Flatpak runtime, instead of making sure that it works against a zoo of dozens of distro, each with slightly different set of library versions, some introducing subtle incompatibilities. It is thus closer to, e.g., what Valve' Steam provides for native Linux executables.
Yes, in an ideal world, you would like that the devs of your distro take the time to custom optimize and adapt the application and integrate them nicely with the specific library versions you have. (And hope that other devs similarly replicate this effort on other distros).
(I am lucky, nearly all what I need is available in this way from repos - so that's indeed how I install it).
But these container apps "built against a fixed base layer" is the next best thing before needing to go to "single app VMs".
It's convenient, but if you have more than 5 to 10 of those packages running on your system at the same time,
The prime target are applications which have a very large collections of dependencies (think large office suit like LibreOffice, rather than some lightweight text editor than doesn't depend on much more that the base Qt libraries).
You aren't very likely to run more that a couple at the same time.
And if you check the applications listed as example:
(e.g. Firefox, Thunderbird, VLC, Spotify, OBS Studio, Google Chrome, Telegram),
These example all support playing and/or recording media, and thus they all would need ffmpeg/libav or gstreamer and a bunch of codecs.
Those things are sensitive to versions.
A distro dev would need to make sure that all of them are compatible with the exact version of libraries shipped in my distro (and patching around any bugs).
Or failing that, in practice lots of distros will ship several different versions of the shared libraries, with differing sonames, and you end up with 2-3 version of all multimedia libraries installed, differing only by the number tacked after .so (that's currently the case on my opensuse laptop).
(This starts to look very close to how docker and flatpak handle this).
Next use case for docker and flathub (and an extra use for conda environment, for that matter): when you just quickly want to test one specific application that you're probably going to delete afterward, but don't want to install a zillion of specific dependencies for it, and are too lazy to also remove all the not-needed-anymore deps once you remove the app.
(I personally use a couple of containers this way. And use conda a lot for testing data analysis).
SteamDeck is another different use for this containers: it makes it possible to install apps while still keeping the root partition read-only, thus it brings a whole catalog of apps to newbie non-fluent in Linux, who would have otherwise needed to switch their root to read-write and thus risk b0rking their SteamOS installation and bricking their consoles. Flathub remove potential user support headaches from Valve, while still giving options to NON-power users. (Would be a shame if you needed to be a seasoned Linux user to add application to your gaming console).