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.
Hehe, that’s called nostalgia. I can feel that, too. 😊 Things used to be simpler but that had some appeal to it. And a different vibe. And you had to work hard. That made your achievements more rewarding than spending your time fighting with complex buildchains.