The total number of primary care physicians in the United States is no more than 130,000 (and falling). The number of prospective patients in the United States is roughly 300,000,000 (the entire population). Current primary care physician hours per patient-year is .5. That means that total hours required is 300,000,000 X .5 = 150,000,000 hours. Hours per primary care physician is 150,000,000 / 130,000 = 1,154 hours per physician. Primary care physicians spend roughly 60% of their time with patients. A man-year is 2,000 hours. 60% of a man-year is 1,200.
This doesn’t take into account issues like maldistribution of primary care physicians (some areas are overserved, some are underserved). This is pretty darned tight.
Conclusion: there aren’t enough primary care physicians to serve the entire population without changing the delivery model. Corollary: primary care physicians can’t take on additional workload.
(Inspired by a comment by TM Lutas)