I think the point others are making is that there is already a surplus of people that want to be doctors and a fixed number of a medical degrees in both scenarios.
If doctors already make more than enough to pay off their medical debt, what would change to make them start accepting lower salaries? It seems that the salary is driven by competition between hospitals (demand ) opposed to what doctors are willing to take. I guess it is possible that doctors without school debt would less actively chase higher salaries, so perhaps there is some small impact there.
That said, it seems by far the best solution is still more doctors, with or without school debt.