Tautology is tautology.
> but Java applets were removed from the browsers
Java applets provided more scope compared to the browser itself, not less. They're not really comparable to seccomp or namespaces.
> hosters who will hand off user account to a shared server
There's lots of CI or function runners that expose docker-like environments.
They are comparable because they provided a restricted sandbox to execute untrusted code.
> There's lots of CI or function runners that expose docker-like environments.
These are running inside VMs.
Java applets were killed off my MS's attempt at "embrace, extent, extinguish" by bundling an incompatible version of Java with IE, and Sun's legal response to this.