“Its single payer from the patients point of view.”
Then why do they need to pay for a Supplemental Plan?
It’s only single payer for the things Medicare decides it covers. (One of the reason not to go single payer. But the flip side is you need a law degree to figure out what is covered by insurance companies)
If you can’t purchase supplemental plans and Medicare says no, then it’s either out of pocket or you’re just out of luck.