And then more and more providers would opt out of taking Medicare at all, so we would quickly end up back where we started.

No, you just make it compulsory to accept Medicare.

Actually I thought that was already the case.

The "just" is doing some heavy lifting there. Clinicians aren't required to participate in Medicare; in fact they have to go through a complex process in order to join. And there's no way that such a mandate could ever pass Congress, or be effectively enforced. Let's focus on reforms that are actually achievable in the real world.

https://www.cms.gov/medicare/enrollment-renewal/providers-su...

Seems to me you're more interested in finding problems than solutions.

Every other developed country has managed it so why don't you look to see how they achieved it?

Wouldn't you like to never worry about healthcare costs again?