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 that make up millions of Americans’ retirement savings.

Such a change would shake the entire health care system, which makes up a fifth of the United States economy, as hospitals, doctors, nursing homes and pharmaceutical companies would have to adapt to a new set of rules. Most Americans would have a new insurer — the federal government — and many would find the health insurance stocks in their retirement portfolios much less valuable.