Second, if the ACA requires 12 million words of regulations to operate imagine how many words of regulations would be required in a single payer system in order to mandate what treatments will be covered, what will be rationed, what medical providers will be approved, etc. I doubt the best and brightest would be interested in entering the medical profession when how they treat their patients might be directed and controlled essentially by government bureaucrats.
Third, actuaries project that when the next generation retires, Medicare will have unfunded liabilities in the trillions. Who knows what the unfunded liabilities would be after a few years of single payer providing health care for over 300 million Americans?
I could go on, but I think I have made my point. Most ideas are bad ideas; ask an inventor. Universal single payer is a bad idea in a Constitutional Republic!