Single payer is extremely expensive. To operate, it requires gobs of money. Money in that quantity, requires a variety of new taxes and at rates that are confiscatory.
Socialized medicine by policy must include restrictions on behavior. It has to. High risk behavior such as poor eating habits, alcohol or tobacco abuse must be heavily restricted to reduce risk to the insurer.
A large bureaucracy must be created to administer a program that will insure over 300 million people. That could add tens of thousands of people to the government payroll. That would come at considerable expense.
Finally, because a system as large as one that would be required to cover 100% of medical expenses it would be impossible to cover every malady for every person. Based on that premise, care would have to be carefully dispenses. People in positions of authority would have to make decisions on type of care. I fear this would be done using impersonal mathematical calculations and actuarial tables.
No longer would the right to life be considered in the highest priority.
Decisions such as would an otherwise healthy 80 year old person with a heart condition be eligible for a stent or bypass? Under single payer what normally would not even be a consideration, my guy feeling is the government bureaucrat gate keeper would simply tell the 80 year old guy, "take these pills. And get your affairs in order. Thanks for contributing. You had a good spin. But it's someone else's turn."....
Don't try to convince me these things are not possible. Bureaucracy is cold and impersonal.