Which groups of people would be exempt from Medicare For All?

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2. Private insurance and private doctors will never be "illegal"

The rich and well to do will always be able to purchase supplemental policies that cover more than basic insurance provided by an all inclusive system. In addition, people will always be able to go to private doctors even if they do not accept that basic system.

Look at how Medicare works now - many people who can afford it buy supplemental plans at various price points. These cover all or some of the 20% that Medicare does not cover. People also pay for private doctors or for elective procedures out of pocket. None of that is "illegal" now or would be in the future if a Medicare like system is set up for everyone in the country.

Supplemental insurance is a huge business and would continue to be profitable for insurance companies.

The ideas you discuss in your OP seem to be right wing talking points against a national health insurance system - fueled by ignorance of how national health care systems work in most advance countries in the world. Please educate yourself before bringing up something you apparently know little about.