Then you can presumably go to another company. There isn't another YouTube.
More generally, unlikely as it is to hit even this comfortable but modest in the grand scheme of things success, it almost certainly won't last indefinitely. Whether because YouTube changes something or because audience tastes change.
None of those are really substitutes for YouTube. But I agree that if someone has stumbled into a somewhat sustainable business that they decide they want to do full-time as long as they can, they should diversify both their content and their channels.
