I mean, the original plan for what became ACA was single payer. They didn't have the votes from the moderate democrats though, which is why the "private plans on public exchanges" became a thing instead.

You're not wrong at at least for the forseeable future, single payer does seem untenable politically, but I'd argue that dismissing studies like this on the basis that it needs solved before debating it on its merits is circular, because the only plausible political objection to a policy like this is financial. If you reflexively claim that nothing without broad consensus appeal at a given point in time is worth discussing, you're essentially arguing in favor of freezing our public policy to whatever the current public opinion is today. I don't think you need to go very far back in history to see some pretty striking examples of why that would be undesirable.