Yglesias isn't scared:
It’s true that any particular doctor can refuse to see Medicare patients and instead fill his schedule with the privately insured, but doctors as a whole can’t do this. Medicare beneficiaries are too big a slice of the market for the entire doctoring industry to give them up as clients. Instead what you’re looking at is that in markets with meaningful competition among health care providers, the most in-demand providers are filling their schedules with the highest-paying customersi.e., not Medicare beneficiaries. Medicaid has even lower payment rates than Medicare and so we already have a quite robust “lower-tier” of health care providers who take Medicaid patients and a higher tier that doesn’t.
Aaron Carroll is much more glum.