Private-equity investors have poured billions into healthcare but often game the system, hurting both doctors and patients.  When a business gets bigger, it forces mom-and-pop players out of the market, but it can boost profits and bring down costs, too. Think about the pros and cons of Walmart and “Every Day Low Prices.” In a complex, multitrillion-dollar system like America’s healthcare market, though, that principle has turned into a harmful arms race that has helped drive prices increasingly higher without improving care.