Beyond that, most Americans under 65 are covered by their employers, and are reasonably happy with that coverage. They would understandably be nervous about any proposal to replace that coverage with something else, no matter how truthfully you assured them that the replacement would be better.

So the Affordable Care Act went for incrementalism — the so-called three-legged stool.

It starts by requiring that insurers offer the same plans, at the same prices, to everyone, regardless of medical history. This deals with the problem of pre-existing conditions. On its own, however, this would lead to a “death spiral”: healthy people would wait until they got sick to sign up, so those who did sign up would be relatively unhealthy, driving up premiums, which would in turn drive out more healthy people, and so on.

So insurance regulation has to be accompanied by the individual mandate, a requirement that people sign up for insurance, even if they’re currently healthy. And the insurance must meet minimum standards: Buying a cheap policy that barely covers anything is functionally the same as not buying insurance at all.

But what if people can’t afford insurance? The third leg of the stool is subsidies that limit the cost for those with lower incomes. For those with the lowest incomes, the subsidy is 100 percent, and takes the form of an expansion of Medicaid.

The key point is that all three legs of this stool are necessary. Take away any one of them, and the program can’t work.