President-elect Donald Trump says the top of his agenda will be repealing the Affordable Care Act, better known as Obamacare. His campaign manager even suggested recently that he might call a special session of Congress to do it. He didn't get into the weeds during the campaign about what he has in mind to replace it with, but he said he believes “every American deserves access to high-quality, affordable health care.” Since his victory, he has indicated that there are some parts of the signature law of his predecessor that he would like to keep — specifically, the provision that allows children to stay on their parents' insurance policies until they are 26 and one that prevents insurers from rejecting coverage for people with pre-existing conditions. One reform he has specifically mentioned is legislation to allow insurance companies to sell policies across state lines.

So what might such a system look like?

Republicans have been agitating for an interstate health insurance market for years, saying it would increase competition and allow consumers to pick plans without mandated benefits they don't want or need. Democrats (and many health policy experts) warn that such a reform would lead to a “race to the bottom” in which insurers shop for the most lenient regulators and attract young, healthy people with bare-bones plans that offer little protection. In turn, those left in legacy plans would be older and sicker, leading to spiraling costs.

But the bigger issue is this: Insurance companies may not even want to do it. Federal law doesn't actually prohibit interstate sales; it's up to individual states, and a handful have tried it. But few if any insurance companies have actually taken advantage of the opportunity. The issue is not just regulatory but whether an insurer can provide the networks of hospitals and doctors necessary to actually deliver care. A Marylander might be able to buy a cheap insurance policy from Texas, but it doesn't help if all the doctors here are out-of-network.

Allowing children to stay on their parents' policies is not a major issue. Many states, Maryland included, passed similar laws before the Affordable Care Act. Since the people in question are young and generally healthy, the costs associated with the provision are minimal.

But that can't be said of the prohibition against denial of coverage for those with pre-existing conditions. The reason insurance companies followed such policies before Obamacare wasn't because they were heartless but because the economics are terrible. If you can get insurance after you've been diagnosed with an expensive health problem, why pay premiums during the years when you're healthy? Once an insurer's risk pool includes only those who require costly treatment and not those who are well, the system collapses.

There are a couple of ways around the problem. You could allow companies to charge much more for those with expensive medical conditions, but in many cases that will amount to the same thing as a prohibition on coverage. If you think the increases in premiums for some policies under Obamacare are large, look out. Or you could find a way to get young and healthy people to buy insurance — for example, by hitting them with a tax penalty if they don't. Which is what the Affordable Care Act does. Republicans have recently been floating a variation on the theme, which is to create a now-or-never open enrollment period regardless of health status, after which pre-existing condition exclusions could apply. But that's not very likely to persuade young invincibles to buy insurance while it's extremely likely to lead to tragic consequences down the line for those who are excluded. There is simply no way to get the goodies of Obamacare without some cost or trade off.

Another question of particularly crucial importance for Maryland is what happens to the health delivery and cost containment experiments authorized by Obamacare, including the state's promising waiver to traditional Medicare reimbursement rules that has shown great promise in slowing the growth in health care costs while improving key measures of quality. If Mr. Trump believes “every American deserves access to high-quality, affordable health care,” he should take a look at what's already working before throwing out the entire system.