It's a good question. As a twenty-something with no major health issues I am really unfamiliar with how the whole healthcare thing works, but here are my 2 cents:

1. Medicine is as broad a term as "Art" or "Business," no? There are going to be people whose job is to follow algorithms and protocols, like nurses, surgeons, (maybe dentists?) and then there are going to be people whose job is to make deciisions about treatment as a whole (idk how they're called, pediatrician? diagnostics? Big Boss MD??) The latter would most likely benefit from a broad range of knowledge and experience (see Dr. House for scientific reference.) Also you have medical scientists who will most certainly benefit from a wider range.

2. There's actually a few chunks in the book where the author talks about how this specialization might be hurting medicine and, in turn, patients. I honestly skimmed over those parts but the idea was that the Throat Cancer MD will look for throat cancer, Left Thumb Cancer MD will look for left thumb cancer, and there's no one who can actually look at a person as a unit, rather than a collection of body parts. So maybe just because our educational system enforces strict (Super) specialization, that doesn't necessarily mean it's the best route

3. "Generalization" in this context doesn't necesarily mean you half-ass three medical "specializations." It could mean something like trying to run a Youtube channel (I see many doctors and psychiatrists doing well on Youtube) or even hustling your way to opening your own clinic so you can do things the way you always thought they should be done. All of that will inevitably require you to gain new knowledge and skills, whether it's speaking to the camera or raising investment