I agree with you. It would be impossible for the critically ill to be in their own insurance pool. There is noway even with massive federal subsidy to be financially viable for an insurance company. An most health insurance companies are consolidating into national corporations. This will limit competition, which will lead to higher prices. An competing across state lines will mean nothing.
The undeniable truth is for profit health care is no longer financially viable, if it ever was. A national health system is the only rational affordable way to deliver health care services to the American public. This means wage and price controls. Allowing for only a small profit for those that provide the products and services in the health care system.