Pure single-payer, regardless of its policy merits, is a political disaster waiting to happen. Government-hating Republicans will hate the idea of increasing the size of the state, the insurance companies and health care providers will fight it tooth and nail for reasons of self-interest, and tens of millions of Americans who get their insurance from their employers will be forced to exchange it, and pay higher taxes, for an uncertain promise of higher wages and a new, but better, benefit package. Skepticism will abound, and not without reason. I just don’t see any way that opposition can be overcome in the foreseeable future.
Permitting people to buy into Medicare, on the other hand, makes perfect sense from both a policy and a political perspective. You can’t plausibly call it a government takeover of health care, because it would be purely voluntary. You can’t say it would drive up taxes or balloon the deficit, because the individual purchasers would be required to pay full price. You can’t complain with a straight face that it is an attack on Medicare or a scary new government program, because it would be an extension of an existing system that enjoys the support of the public.
In short, it would expand coverage at a more acceptable price and have a reasonable chance of getting through Congress, even though it would be opposed by some Republicans and providers and by the insurance companies. I expect it to be included in the Democratic platform in 2020.