Who would want to use those weird, outdated languages and technologies, though?

It's better to use a modern DevOps stack involving Kubernetes, Ansible, YAML, Jinja2, Python, cron jobs, regexes, shell scripts, JSON, Jenkinsfiles, Dockerfiles, etc.

Yep. At this time it wouldn't be great to just imitate the best mainframes compartmentalization, because we have much better experimental systems. But they are still much better than anything common on commodity hardware.

Likely because IBM mainframes are outside of the reach of hobbyists and SMEs.

No, it's because the "capabilities" built into mainframes aren't accessible by users, only the system administrators. The granularity just isn't there.

That too. But I'd argue that not being able to access or set up a mainframe due to its unaffordability or esotericism is a much more powerful barrier, that makes granularity a moot point.