A person with medical qualifications once chose to pick up garbage for a living instead of practising medicine. When interviewed they said that they saved more lives in a month by working as a sanitation engineer than they would during their entire career as a doctor.
That person doesn't understand marginal returns / replaceability. They could have been a doctor then paid some homeless person a small amount to pick up garbage for a living, easily combining both outcomes.
Considering that there are many more qualified willing and able candidates trying to become doctors and many less trying to become sanitation engineers, it actually may be a larger net benefit since the next medical student in line would likely be very close in ability.
