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.
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.