UMass Memorial Health CEO Eric Dickson has a grim diagnosis for the state's health care system: Without a dramatic change, more closures will likely follow the recently shuttered Carney and Nashoba Valley hospitals. Those two hospitals went dark in large part because of their for-profit ownership under the now-bankrupt Steward Health Care, which undermined their future by selling their real estate. But the message from the Worcester hospital chief to a small crowd of business leaders at the Chilton Club in Boston was sobering: Nonprofit hospitals in Massachusetts could soon face similar fates.