Greatness is not the same as usefulness.
If the janitor is as necessary as the CEO (otherwise the washrooms would stink and the trash cans would overflow), then why is the latter paid more?
While people should be allowed to pursue whatever it is that makes them happy no matter how supposedly “inferior” that is, I think there is still an objective criterion for greatness. The janitor, while necessary to ensure that the whole corporation runs smoothly (for nobody wants their toilets to stink) is paid less than the CEO because s/he contributes less to that corporation’s achievements. The janitor’s tasks are less critical and require less skill than the CEO’s.
Achievement is still the objective criterion for greatness.
Achieve. Be great. Or else be ordinary. (Useful – yes! – but ordinary.) It’s your choice.