Not OP, but I'll answer.
Switching to a single-payer will be too costly for any political party to tolerate. The US spends $3.8T on health care per year. That's $11,500 for every single person in the United States.
In order to fund (raise taxes) to a single-payer system that number will need to drop dramatically. We'll have to cut out middlemen, reduce bureaucracy, negotiate domestically, etc. That $11.5k needs to drop to about $6k to be on the upper quintile of health spending per capita compared to other, successful, single-payer systems. That brings the total from $3.8T down to $1.98T.
...but therein lies the problem. The health sector is 18% of the economy and employs 14% of all US workers. You can't quickly pull $1.8T out of a single sector of the economy. Would it stabilize to 9% of the economy and 7% of all US workers? What do all those people (7% of all workers) who previously worked in health care go do? Where does that $1.8T that we used to spent show up?
So it's a conundrum. You need to slowly reshape the industry over 10-20 years and carefully manage the outcomes or it's an economic and political disaster. However, the only way for that gradual plan to work is if you raise taxes to fund the $5T that it would take to maintain status quo. ...and good luck with that. The total net worth of all the billionaires in the US is $5T. There isn't a tax rate in the world that could raise that kind of revenue.
Vermont tried to implement a single-payer system, did their own analysis of what it would take, and decided not to move forward.[0]
So it's not that I oppose single-payer in theory, I just oppose it as policy in the US because I see it as a political impossibility that is in the way of realistic reform. To me it's chasing a perfect outcome when there are good outcomes available.
[0]:https://www.vox.com/2014/12/22/7427117/single-payer-vermont-...