The insurance companies would have to transition. Lots of people work for and serve this industry. All-inclusive public health care would destroy this industry beyond recognition, and those people would have to find other work.
Hospitals would have to transition. In many small cities the local health care system is the biggest employer. As Reihan Salam points out in The Atlantic, the United States has far more fully stocked hospitals relative to its population and much lower bed occupancy than comparable European nations have.
If you live in a place where the health system is a big employer, think what happens when that sector takes a sudden, huge pay cut. The ripple effects would be immediate — like a small deindustrialization.
Doctors would have to transition. Salary losses would differ by specialty, but imagine you came out of med school saddled with debt and learn that your payments are going to be down by, say 30 percent. Similar shocks would ripple to other health care workers.