ACA eliminated the cap on the maximum insurance would pay. In the typical 'single payer' system, like medicare, which is termed 'single payer' but is actually a dual payer system with optional third party payer. The way these systems are intended to work is entitlement contributions pay for a major fraction, and the patient pays for a lesser fraction, say 20%, and the patient has the option of taking out insurance to cover the 20%. When everyone is covered there are no unpaid hospitals (in theory).
The situation in the U.S. is unique among developed countries. Because the U.S. system amounts to a blood sucking cartel, before the ACA, all manner of measures against the sucking arose in the form of ungodly amounts of paper work piled on top of ungodly amount of government regulations and still more paperwork (one third, or more?, of the cost of care is paperwork cost), hospital admission pre-clearance, 'out of network' stuff, pre-existing conditions stuff, out patient-in patient stuff, pharmaceutical tiers, approved supplier stuff, and on and on and on, blah, blah, blah forever, and ever, amen. And the ACA only managed to do away with some of this. Unbelievably high costs, 7$/ box of kleenex, 15$/aspirin tablet, etc. resulted in astronomical bills and rampant bankruptcies. Even with insurers paying 80% of costs, 20% of a quarter of a million was difficult for some families to come up with. And the rates for supplemental insurance to cover the additional 20% were (are) high.
So the real problem is out of control costs. Any proposal that doesn't address cost in a serious way won't do much to solve the healthcare problem in the U.S.
One approach that has been proven to work is coercion. The two main economic classes in modern social democracies are Capital and Labor. While the American medical cartel serves Capital splendidly, from Labor's point of view it sucks. Capitalism does not work for Labor unless there is a free market. And pure free markets, other than on a local scale, seldom exist in modern America, and they don't exist at all in U.S. healthcare. Denying this is like denying that the Earth is round.