You seem to think that insurance companies are driving the price of healthcare, and not providers. Insurance prices are set basically like this:
insurance premium = estimated cost of providing care (that is, the fees charged by healthcare providers) + taxes + admin expenses/profit
If you have a giant hospital system dominating a metro area, what do you think would lead to lower prices:
1) One or two giant insurance companies negotiating with the hospital on behalf of the majority of the residents of that metro area
2) 15 different insurance companies, each with a few thousand members in the area negotiating rates with that hospital?