Single payer means what? It does not mean a single provider. It means the government provides medical insurance coverage for everyone — Medicare or Medicaid for all.
Does that also mean that everyone with employer-provided insurance loses it? If so, then a wide range of people from unionized workers to executives, everyone with high-quality insurance, will not be happy when down-graded to the one-payer plan. That will never happen; it would be too unpopular. As well it will be politically impossible to tinker with Medicare.
If those are all valid conclusions, then single-payer really means government as the insurance provider of all who do not have medical insurance from another source. This raises a hundred questions. Will the government provide this insurance for free or will it charge for it? Will the overall rate be adequate or will subsidies be required? What gradations will be allowed? Will there be an individual mandate? Will insurance companies be required to cover those with pre-existing conditions or will they all fall into the government insurance plan?
Before getting too excited either way, the structure of a single-payer plan needs to be better defined. One version may not be all that different from ObamaCare without exchanges.