Are you referring to this Nix effort or to Docker? Because that largely applies to most usages of Docker.
Saying that Docker could be replaced by a simple script that does chroot + ufw + nsenter is like saying that Dropbox could be a simple script using rsync and cron. That is, technically not wrong, but it completely misses the UX / product perspective.
Saying that nix is a simple script that does chroot + ufw + nsenter is missing the point even more
Glancing over the UX/DX criticism is also missing the point. So I guess everyone in this thread is just looking at their own little world.
I'm saying that all usage of docker that I have ever seen is as a laughably over-engineered schroot. Literally. It can probably do more stuff, but I haven't seen it.