I don't think medicine is a junk investment when you consider the opportunity cost, at least in the US.

Consider my sister, a fairly median medical school graduate in the US. After 4 years of medical school (plus her undergrad) she graduated with 150k in debt (at 6% or so). She then did a residency for 3 years making 50k a year, give or take. After that she became an attending with a starting salary of $220k. At younger than 30, she was in the top 4% of salaries in the US.

The opportunity cost is maybe ~45k*4 years, 180k + direct cost of 150k or so.. So $330k "lost to training," however 35+ years of making 100k a year more than some alternative version that didn't do medical school. Depending on investment and loan decisions by 5 years out you've recouped your investment.

Now, if you don't like medicine and hate the work, you've probably damned yourself to doing it anyway. Paying back that much loan is going to be tough working in any other job. But that is a different story than opportunity cost.