I'm not sure what the argument is about.
The United States already has an effective, working system of universal healthcare. It's called Medicare.
It covers substantially all the population, without regard to ability to pay, pre-existing conditions, or anything of the sort. All a person has to do is meet the age-based enrollment requirement. I'm not sure what the policy justification is for the extremely high minimum age requirement, but we have age requirements related to voting, the consumption of alcohol and tobacco, driving an automobile, and so on, so I don't have any problem in the abstract with an age requirement.
If the opponents of universal medical coverage had any political courage or intellectual integrity, they would be working hard to repeal Medicare and get the government the hell out of the health insurance business. If the supporters of universal medical care were clever, they would be working hard to repeal (or at least lower) that minimum age requirement.
But, no, we are all knotted up in our underwear not quite sure what to do with an incoherent health care delivery system that perversely expects private for-profit entities operating in a free market to ration medical care in an efficient, fair and politically acceptable way. Wrong tools for the job, fellas.