effective, altruistic career whatsoever because the shear supply of people interested and capable of becoming doctors is so great (even after artificial restriction).

I'm not sure whether the "even after" makes sense in that sentence. There are a lot of interested and capable people applying to med-school If you get into medschool that means that one of those people won't get into med-school.

On the other hand if you become a skilled programmer you don't take a job away from anyone. That's why 80,000hour recommends people to rather become a programmer at a startup then to study medicine.