Also, people hold their bind mounts to the host near and dear, and user namespaces would break all kinds of things people expect to "just work" with bind mounts

Also, people hold their bind mounts to the host near and dear, and user namespaces would break all kinds of things people expect to "just work" with bind mounts. Having user namespaces on by default would break tons of existing scripts, Compose/Kube files, etc. that do things like mount /var/lib/mysql into the container for persistence.