Mir, Unity, now Snap. Ubuntu has a track record of wanting to go it alone. But, I'm all for competition, long may it continue. The only real negative here is that some apps will only release Snaps, others will only release Flatpaks and people will end up having to just revert to copr/AUR like before. Choosing a distro is basically choosing a DE and package manager these days anyway - a single unified packaging format…
I've had some success with Nix. Still I would only generally recommend it for development (even though I use NixOs)
Through nix-env it even already has the idea (albeit discouraged) of imperative package management like apt.
Having spent a while with NixOS, running containers for user applications seems like using a cannon to kill a fly. The 'simple' alternative is to drop FHS altogether - which containerization is kind of doing in a roundabout way by isolating the app filesystem from system fhs.
As for it being for developers only... I get that perspective. Debian/Ubuntu packaging is also hard, AUR packaging has its quirks. A lot of this is hidden behind the package manager, wheras with NixOS it is more obvious.
The killer idea for NixOS would be to make package management for Joe Q Public as easy to use as apt while. Tools like MyNixOS[1] are emerging which might bridge that gap.