> after service has been rendered can decide to pay less, or not pay at all, or stop paying that provider altogether, etc

This actually isn't necessarily true when you learn how billing codes work. Most insurance companies pay out at a fixed rate per billing code based on your plan. That doesn't change. What does change is that hospitals can retroactively apply new additional billing codes.

This happened to us once for an ER visit where we got 3 additional surprise bills over 6 months because the hospital retroactively applied new billing codes to our visit.