Not shown: "systemd-run --shell" asks for the root password (or whatever set up for polkit authentication). That some other process of the same user can influe

Not shown: "systemd-run --shell" asks for the root password (or whatever set up for polkit authentication). That some other process of the same user can influence that session with elevated privileges isn't really an issue. In fact, the terminal in which systemd-run was invoked is also owned by the user and can be "taken over" in the same way.