"When were insurance markets ever driven by supply and demand models?"
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.....are you serious?  If I don't want Gieco, I can choose All State. I can choose USAA. I can choose Farmers. All of these insurance companies promise me lower prices and better services.  What on earth do you mean that's not on a supply and demand model?   Now look at medical coverage: I am on an Obamacare coverage and 'MY' doctor won't take it.  Why on earth do you think the California Obamacare centers and networks are evaporating?  No one is joining and insurance companies are leaving-- that is precisely because there is NO MARKET, anymore.
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"Before the ACA insurance companies cherry picked their customers." -- False.  Go look up 'adverse selection,' people at risk get policies all the time without Insurance companies knowing about it.  Of the ones that are riskier, they pay more-- THAT is what makes it INSURANCE.  Pooled with enough people, that drives prices down for everyone. This is ECON 101.