Also, the US doesn't really have too few doctors. Maybe we have 30% fewer than the typical OECD country, but we also have far greater mid-level penetration and typically work under harsher conditions with longer hours. The US has poor distribution. Too many specialists. Too many docs in urban areas. Doctors also work more hours and see more patients in the US, especially when you consider residency. So US citizens are more than getting their money's worth from what they pay for doctors, that 8% is very efficiently spent. Administratively, obviously there's is massive waste, but I don't really think this article is making any salient points based in actual data or real life experience.
In the US if you need life saving care you get life saving care. Exceptions might be if you live in rural North Dakota, have a STEMI, and are far from any facility that does PCI, but this happens in rural communities all over the world. In the US if you need elective care you get elective care, and far quicker than you would in other countries.
More doctors does not equal cheaper care. It's 8% of the pie. As others have said, you can halve doctor pay overnight and we're still paying 96% of our current healthcare costs. More realistically, you could massively expand the number of NPs and replace many physician tasks, but on a per-patient basis NPs cost nearly as much as doctors and represent a massive liability in a system that values safety above all else.
This is The Atlantic, so typically they back up their points pretty well, but the best he can come up with is some tangential arguments. There is nothing here that actually makes a connection between better healthcare outcomes and more physicians in the US. Just "the US has fewer physicians" (okay, and...), "schooling is unnecessarily longer" (I agree, but this proves nothing), and finally "it's basic economics!" (so why don't you... show the economics).
I'd be willing to bet that this guy set out to write this with the intention of saying, "we need more doctors and lower pay for doctors, and here's why." The problem with the US healthcare system has never been doctors, so he found nothing substantial and wrote a fluff piece instead.