Maybe things have changed in the last two years, but this was my understanding:

> One percent of all non-pediatric physicians have formally opted-out of the Medicare program in 2020, with the share varying by specialty, and highest for psychiatrists (7.2%).

https://www.kff.org/medicare/issue-brief/how-many-physicians-have-opted-out-of-the-medicare-program/

When I was a practicing attorney, I assisted seniors with figuring out their health care options. I can confirm that Medicare Advantage is almost always a bad idea. It might seem like a better option at first, but they scam you in ways that aren't readily apparent during enrollment (frankly, every private insurer is trying to scam you).

• Prior authorization - most advantage plans require preauthorization before allowing you to receive care. This is a key point where they will try to screw you over even though your doctor may actually disagree with their determination.
• Networks - advantage plans have networks like a typical insurance plan. Traditional Medicare does not - most providers will accept Medicare. Very few have opted out. Good luck finding skilled nursing care that is covered and in-network on your advantage plan.
• Profit motive and administrative overhead - come on. They are trying to make a buck and are far less efficient than traditional medicare.