I think it's telling that people are shocked at the assertion I just made, which is not complicated or outlandish or hard to understand and is in fact backed up by referendum and attempted implementation results for state-level programs. I think two big things are happening that fog people's understanding of this issue:
First, there's a widespread belief that M4A is popular, based on public opinion polling. The problem is that you can make almost anything popular in public opinion polling, and a lot of public opinion polling is deliberately run by interest groups to generate narratives about popularity. It's true: the "M4A" that poll respondents support would be enormously popular: it's proposed as abstraction with no clear tradeoffs. When you confront voters with the prospect of increased taxes and the loss of their current insurance policies, the wheels come off the wagon.
The second big factor is that the demographics of people with employer-provided coverage --- the majority of all non-Medicare covered people in the US --- are not what you'd expect. As soon as you stipulate employer coverage, the cohort you're describing excludes basically all fixed-income and Medicaid-eligible households. The median household income of a family with employer-provided health insurance is closer to $120k than it is to $50k.
For those households, M4A is not a very compelling deal:
* There is a very clear trend in the data for them to already be satisfied with their existing health care.
* The visible component of their insurance spending (their out-of-pocket, excluding employer side payments) is usually quite small compared to total spending.
* M4A would mechanically eliminate the availability of existing plans (unless you came up with a truly weird and distortionate system of tax incentives to keep Anthem and United and Aetna policies going).
Best case: costs that are hidden from those households today become visible, and you hope people are chill about that (in sort of the same way we hoped that people would be chill about inflation given wage increases outpacing it --- see how that went). Worst case, a lot of these households would lose their existing, favored insurance plans and pay more.
Useful here to note that broad taxes on the middle and especially upper-middle class are how Europe funds generous social service packages; you can't get there by taxing the bejeezus out of billionaires. You should do that anyways, just because it's a good idea, but there aren't enough of them to pay the absolutely gobsmacking cost of a single-payer health system in one of the wealthiest large countries in the world.
I'll cop to this: what I wrote last night, about "currently insured" people, was way too vague. I should have said "households with employer-provided health coverage" (again: that's most non-Medicare households). I plead strep throat; you're going to have to give me a break on clarity today.