I'd disagree. Please add in 6 years of income for the fast-food employee and deduct 6 years of expenditure (plus ongoing qualification fees) for the medical doctor.
Just as you assume that medical doctors WILL become specialists, please consider that those who start off as fast-food employees will also learn on the job. Many of them will move on to become managers in the food-services industry.
Perhaps change the statement to 'unless you can afford the education for a medical degree'...
Few if any fast food service manager can earn 30k in an afternoon whereas many procedural specialists can. You don't need to work that many afternoons to pay off the HECS.
The costs and trials/tribulations of getting to become a specialist is well worth the effort, financially. Just like climbing a mountain - once you get to the top, the view makes it all worthwhile and you forget the pain of the climb.