Isn't that basically what they're doing with Obamacare premium subsidies? And wouldn't your plan be even more expensive?
Put people with pre-existing conditions in a sort of "high risk" pool like they do for auto insurance, and have the insurance companies share the risk in covering those folks too.
As states found out, "high risk" pools are very expensive. The premiums were prohibitive, the coverage was very limited, and that's why many states never got into the high risk pool business to begin with.