The public pays for coverage and expects to be covered. If the insurers think the cost is too high then they should just increase premiums instead of denying coverage under existing contracts. Simple like that.
On single payer, very complex procedures like organ transplants, tumor resections in complicated places (ie neurosurgery) become completely out of reach of normal people. That is also extreme and undesirable.
See, there is a qualitative difference there. Not being able to pay single payer for a procedure that is way too
expensive is one thing. The other, is paying for insurance coverage and the insurer acting on bad faith to not deliver what the contract said it would.
The latter is just fraud but in America the judiciary lets this happen as it is dysfunctional as well.