There are countries that have single-payer systems and widespread supplemental insurance. But if you universalized Medicare, you'd immediately do at least two big things to the market:

(1) You'd eliminate the system of advantages and supports that cause employers to offer private insurance, which is where most people get their insurance from.

(2) You'd create a huge adverse selection problem --- the more effective/useful Medicare is, the fewer families will want to spent $24k/yr on private insurance, meaning the families left on private insurance have a reason to want it, meaning the composition of the risk pool would shift dramatically.

Like, if we ever did M4A, we'd probably end up with a widespread system of supplemental insurance; we already have it with Medicare! But that's not the same thing as keeping your existing plan.

I don't understand the obsession some people have with keeping your existing plan. Lots of people can't keep there plan under the current system. Insurance companies update their plans regularly. Sometimes they remove plans or exit markets entirely. An existing plan will get small changes over time. If Theseus has an insurance plan for 10 years and the insurance company makes changes every year can we still call it the original plan of Theseus?

If M4A plus supplemental insurance gives me about the same coverage I have now for a reduced total cost that sounds like a win to me. Even if it ends up costing me the same amount the net improvement from everyone having access to basic health care would still be a win.

Every policy is easy to enact if you just define away anybody who'd object to it. But, more importantly: it's unlikely that M4A by itself (let alone with the supplemental plan you'd likely end up with) would reduce your total cost!