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.
I would not call it a next step. Just another option. The big downsides include much larger on block storage size and worse yet much bigger memory foot print since your app cannot benefit from shared images. Worse system integration too.
In a world where block storage is huge and cheap and memory too maybe less important. I would not say it is without issues though. Maybe convenient but not optimal in a lot of ways.
From my understanding, Flatpak is built on top of OSTree, which will automatically deduplicate files across different packages. That said, I’m not sure if this extends to downloading packages. The site claims that it does do “delta updates,” which would hopefully mean that it doesn’t download files that are already on the system, even if they’re part of another package.
I’m just going off what I read in the docs. Someone with more understanding of the system can clarify.
Flatpak does try to account for storage size by using shared base images. The main problem is that some Flatpak apps don’t update to the latest base, and some use different base images altogether, meaning most of the time it needs to have several bases anyway.
If people want the Linux desktop to become more ubiquitous in homes, it better damn well be the next evolution. Someone’s grandmother isn’t going to get on the command line when apt inevitably decides to break.
The concept is not new, and Apple has had .app containers for a very long time that almost always just works. So clearly the concept has long been proven.
Perhaps flatpak, snap, appimage aren’t the final forms of this concept on Linux, but it’s a step toward making application packaging and distribution much more friendly for the common masses.