But there are lots of situations where you can't do things even with root access on a unix-like machine. I gave one example already -- root access doesn't give

But there are lots of situations where you can't do things even with root access on a unix-like machine. I gave one example already -- root access doesn't give you control over the operation of system firmware. But there are others, too. For example, root access doesn't let you write to kernel memory, it doesn't let you bypass SELinux policies, etc. The way you are defining root access here essentially doesn't apply to any modern computing system.