Greg Mankiw is concerned that a subsidized public option would be unfair to private health care providers:
But that's not really the goal. The goal is to provide the most people possible with the best possible health care. There's no constitutional obligation to make sure no one is "unfair" to private health care providers. In any event, according to critics, government-provided health care will lead to a parade of horribles, even with subsidies. If those criticisms prove true, people won't opt for it anyway.
I assume what Mankiw is alluding to is that people justify the public plan by citing the need to keep private providers "honest." The idea is that private market has not -- and will not -- provide an acceptable level of services without this competition. But Mankiw seems to be using this argument to make the conceptually distinct argument that private providers, in turn, shouldn't be subject to supposedly "unfair" competition (i.e., publicly-subsidized plans). But the latter doesn't follow from the former at all.
If the country opts for a public plan, and people vote with their feet because they like it better, why should we care?