Many people seem to forget that NetBSD's pkgsrc exists and supports Linux among other systems.
It has minimal dependencies compared even to portage, which makes it integrate well with lfs, kiss, or suckless distros>>11964>Is going from distro repositories to this type of stuff going to improve software distribution on linux ?
The problem flatpak, snap and nix are trying to solve is reproducibility.
Source-based package managers usually ensure all dependencies with correct versions are available.
Nix is more sophisticated in its package configuration and exposing a filesystem with exactly the needed dependencies to a compiling project.
Binary package managers nowadays only distribute dynamically linked executables and libraries, that often need to have a particular version to function together. Dynamically linked libraries are rarely compatible between different version, because even trivial bug fixes can change the ABI. Compiling all dependencies yourself would only fail after an API change. Many distros invest a great deal of effort into keeping the repos of their binary package managers working. Binaries distributed for those distro not included in the repos, especially proprietary ones, will inevitably stop working, unless always they get updated in sync with the particular distro.
I don't know about snap, but flatpak has several very general dependency collections ("runtime") an application requests a specific version of. The application is then granted access to all dependencies in its filesystem and optionally user specified directories. Applications and runtimes are distributed as compressed filesystem images on flathub or private repos. Thus flatpak applications should only stop working when the underlying distro breaks flatpak, bubblewrap, dbus, x11, cgroups, … not Gnome or KDE though.
Both types of software distribution are without a doubt less error prone than previous approaches. Still i don't think the latter method will solve all portability problems. Some programs are inherently fragile outside of their target environment, so i have seen even their flatpak applications fail for undetectable reasons.