Back in the late 70's when I left school if you wanted to be a doctor or a pilot the barrier to entry to these occupations was about equal, controlled by high academic requirements in the case of doctors and scarcity of opportunity for pilots. (P2F effectively still then existed in a minor way as the old self improver route via instructing. You had to have a substantial amount of money to learn to fly in the first place, followed on by paying for the various tests and exams along the way.) What changed was the 'scarcity of opportunity' in aviation and the explosion of flying jobs available first in Europe, now worldwide. If you look at today's school leavers the barriers to becoming a doctor are still largely academic. (Anyone who questions this should have a quick look at the BMAT and UK CAT tests required by many UK Med Schools, they are seriously challenging.) The academic barriers for pilots were 3 decent A Levels (the modern equivalent is a half reasonable degree), but what has changed is the surplus of people having access to huge lines of wealth or credit. So moderate academic requirements + ample availability of money = over supply of qualified candidates. Result continuous lowering of Reward.
Largely, I think the complaint that we have lost pay parity with the medics has been overstated. If you look at pay rates for Consultants/Registrars/etc down to 1st Year Post Grad doctors there is reasonable parity with Training Capts/ Line Capt/ Fo's in a legacy carrier. This diverges downwards if you take in the fixed pay rates offered by many LoCo's, but there are similar downward pressures on many of the less glamorous medical positions. Even the hardships of being a P2F cadet for a couple of years can be financially balanced by the 5 years a med student spends not earning and the 2 post grad years of poor pay.
The one big difference I can see is that the Medical Profession have been much better at working as a cohesive unit to maintain their Terms and Conditions at an appropriate level. Mainly this is achieved because there has always been a scarcity of supply of doctors with an ever increasing demand, but it would be interesting to observe how T&C's would change if the high academic barriers were lowered and Pay 2 Doctor was invented. I suspect the results would mirror our own predicament very quickly.