Public support for single-payer health care has been rising in recent months amid failed Republican efforts to repeal and replace the Affordable Care Act.

That’s perhaps why Sen. Bernie Sanders on September 13 introduced a new version of his single-payer plan with the support of 16 Democratic colleagues, a sharp rise from 2013 when none signed on to a similar proposal. It would not only expand Medicare to all Americans but make it more comprehensive by covering more services like mental health, dental care and vision, all without co-payments or deductibles.

But Sanders’s plan would come at a steep price: likely more than US$14 trillion over the first decade, based on an estimate I did of a previous version.

There is, however, a simpler and less costly path toward single-payer, and it may have a better chance of success: Simply strike the words “who are age 65 or over” from the 1965 amendments to the Social Security Act that created Medicare and...