At the heart of the “Medicare for all” proposals championed by Senator Bernie Sanders and many Democrats is a revolutionary idea: Abolish private health insurance. Proponents want to sweep away our complex, confusing, profit-driven mess of a health care system and start fresh with a single government-run insurer that would cover everyone. But doing away with an entire industry would also be profoundly disruptive. The private health insurance business employs at least a half a million people, covers about 250 million Americans, and generates roughly a trillion dollars in revenues. Its companies’ stocks are a staple of the mutual funds...