If an insurance denies care that is contractually agreed upon, the legal action is possible for the person. As Stinkcat regularly points out, government will be under no obligation to provide anything, even if promised.
At least with for-profit corporations I can walk away with my money. Under a government dictated plan, even if I can walk away, government keeps my money.
As for association, the irony is that is exactly what proponents of single payer rely on: a large pool of people required to pay in in order to help cover the cost for the expensive individual. On that we agree. I just disagree that the central government should be in charge, because then all decisions are made by government.
It is association, but not free.