I don’t understand what problem they are meant to solve. If you have a FOSS piece of software, you can install it via the package manager. Or the store, which is just a frontend for the package manager. I see that they are distribution-independent, but the distro maintainers likely already know what’s compatible and what your system needs to install the software. You enjoy that benefit only through the package manager.
If your distro ships broken software because of dependency problems, you don’t need a tool like Flatpak, you need a new distro.
Updates in flatpak tend to be smaller in flatpak than with packages. Flatpak only downloads files that changed, packages always download all files. But yes, the initial install of the first flatpak is going to take more bandwidth. The second package is then already reusing a lot of files already installed by the first.
All the base libraries are ahared between all flatpaks. Only very specialized libs end up getting installed several times (or libs with a different build configuration), but those are very like going to be used by one application anyway, independent of how youninstall the application.
I trust developers to have a way better idea what the software actually needs than some random packages that also packages 200 other things.
Yeap, static linking cannget similar results, but at the price of more bandwidth usage (you need to download the same library code over and over again) and resource usage: You do not get deduplication of libraries, so that code needs to be loaded into RAM repeatedly.
I tried that: It is a huge pain. You basically can not use anything newer than 3 years or so as distributions will not have the new code yet. It is also eating a lot of time when you get but reports and then find out some packager has shipped your code linked to a library kniwn to be broken. Happens all the time:-( You need to be able to point people to working binaries of your product, so you will just package the stuff yourself. Doing that once properly for flatpak is very nice – oder doing it a dozen times, each time testing with the package versions and patches applied on top of that version the distribution ships.
But younleft out the one thing that makes me prefer flatpak: I can restrict what an application can do. E.g. no broser can access anything outside the downloads folder, etc. I care about my stuff in the home directory. Traditional Unix security has nothing to help me stop one application messing with the data from another, stealing my ssh keys or turn on my cameras or microphones. Flatpak can limit all that to certain applications – and when thebapplication supports it, even more than that.