Uh, grades.

You take Doctor 101 and get a C-, well the number of students who graded A-B filled the Doctor 102 class. Study up, and either retake the class or take a test to prove you know the information. You scored high enough on your test? Rad, welcome to the class. This is actually what we do anyway so, you're overthinking things there.

Number of jobs is a weird limitation for gatekeeping professionals. If we only need X amount of doctors, then we're an entirely healthy world with zero illness and no room for new minds to create entirely new methods and further our understanding of medicine? I want anyone driven enough to practice medicine to do so, it's the only way we'll have enough doctors to fix the massive healthcare deficit we're experiencing. Especially through the above grading methods I suggested.

As for the pay decreases, hard to say really without doing it. If an employer believes your education is less valuable because more people can achieve the same, they're a shitty place to work and they'll get what they pay for. There's also the possibility of those doctors being more affordable actually expanding the availability of healthcare overall.

I get why it's worth questioning, but it's broken now so why can't we try to fix it? What if the fix works? Awesome right? What if the fix doesn't work? Good thing the current broken system could act as a fallback, right?