That's true, sort of. I mean, that isn't the whole point of sandboxing because the threat model for sandboxing is pretty broad. You could have a process sandbox just one library, or sandbox itself in case of a vulnerability, or it could have a separate policy / manifest the way browser extensions do (that prompts users if it broadens), etc. There's still benefit to isolating whole processes though in case the process is malicious.
> (And no, most linux namespace stuff does not require root, the few things that do can be provided in more-controlled ways. For examples, look at podman, not docker.)
The only linux namespace that doesn't require root is user namespace, which basically requires root in practice. https://www.man7.org/linux/man-pages/man2/clone.2.html
Podman uses unprivileged user namespaces, which are disabled on the most popular distros because it's a big security hole.