Demonstrators supporting Medicare-for-all rally outside PhRMA headquarters in Washington last week. (Win Mcnamee/Getty Images)

TO HEAR Sen. Bernie Sanders (I-Vt.) say it, one would think that creating a single-payer, Medicare-for-all health-care system is easy. Every other developed country in the world guarantees health coverage to its people, the democratic socialist argues, and so can the United States.

If the United States were creating a health-care system from whole cloth, Mr. Sanders would have a better point. But in reality, Medicare-for-all is not the only way to get to universal coverage; it is just the highly disruptive one Mr. Sanders prefers. In doing so, he deeply mischaracterizes how difficult adopting it would be. A Congressional Budget Office analysis released Wednesday underscores why.

There is no doubt of the need for a more inclusive health-care system. The CBO reported that 29 million people under the age of 65 lacked health insurance last year. Even accounting for uninsured undocumented immigrants, of which there are several million, the number was still far too large for the wealthiest country on the planet. Meanwhile, the nation as a whole devotes $3.5 trillion a year, split about evenly between private and public spending, on health care. That’s about a sixth of the economy. It is fair to expect better outcomes given the size of the national tab.

But another way to say it is that upending such a large portion of the economy would be extremely hard. The CBO’s nonpartisan analysts found the consequences “difficult to predict” because all they had to rely on were experiences from “previous changes that were much smaller in scale.” One thing that was clear: Many of the massive numbers of Americans employed under the current system would have to find new jobs. Under a Sanders-style system, the employer-based health-care plans that most non-elderly Americans use would also be eliminated.

To many, this huge shift in Americans’ lives would be worth it. But that would only be true if many things went right. Administrative costs would probably decline, but a lot of the efficiency of a universal system would be in integrating electronic records, which could happen slowly or not at all. Paying doctors and hospitals less could drive down national health-care spending, making the reform relatively affordable. But doing so could also shutter smaller or regional facilities whose margins are already low. It could also discourage talented people from entering the medical profession and result in long wait times for care. Offering a generous program — Mr. Sanders wants full health coverage plus vision and dental with no co-pays or premiums — could encourage people to use far more health-care services, driving up costs.

Combining low doctor payments and generous benefits, the CBO report says, would create a cascade of problems: “a shortage of providers, longer wait times, and changes in the quality of care, especially if patient demand increased substantially because many previously uninsured people received coverage and if previously insured people received more generous benefits.” Many might be willing to sacrifice the access and perks insured Americans now enjoy in return for cutting administrative hassle and covering more people. But that is not the future Mr. Sanders is selling. He describes a system with practically no trade-offs. Such a system does not exist.