You are making assumptions here and they are betraying biases you may not be completely aware of. For example in the UK (and numerous other countries I am not going to list them all) private and public insurance can coexist and not the whole entire market suddenly decides to drop Medicare.

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For example you assumed that Medicare buy in will be a high risk pool and nobody without pre-existing conditions will buy health insurance that does not have a pre-existing condition. People will want to buy health insurance even if they do not have a pre-existing condition. People will buy health insurance from a private or public source based off cost and benefits, many people do not have an ideology that assumes private is automatically going to be better, or public is automatically going to be better.

Have you ever consider that a medicare buy in will also have employers who will buy their employees in compared to private insurance?

You can't just assume that a medicare buy in will turn into a high risk pool. Furthermore even if the youth has a pre-existing condition covering a younger person that is 18 to 64 is often cheaper than a 65+ person. Sometimes a person with a pre-existing condition that is younger will be more expensive than an older person generally and sometimes this is not the case. Furthermore older people have more pre-existing conditions generally so we are also comparing not a healthy old person to an unhealthy young person in a binary fashion but instead dealing with a dataset that is far more complex.

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Furthermore if you think only high risk people will join a Medicare Buy In (which is a huge assumption I have already dealt with), depending on the details of the plan in the 2nd or 3rd year when the costs for the buy in patients goes up based off the numbers of the 1st year, the buy in cost should go up accordingly.

How is this different than the current status quo where patients either can't have insurance due to cost, or they do have insurance but they are put in an unfortunate situation where costs are keeping on going up? Where is the harm of having a new version of the status quo which is similar to the existing status quo? Under your logic Damookster we did not harm anyone we just created an arbitrage that no one really win or loss we just have a public system that mirrors the private system.

But the possible gain for this "risk" to cause a change is your great fear may not occur. From a betting / odds perspective you should take this change.

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The details of these plans matter, and assuming you know the details will be good or bad without seeing the details is ideology speaking. You are not really thinking or evaluating, instead it is "reflex" aka a form of "habit" without true analysis.

"The devil is in the details", but also simultaneously quote Not all angels reside in heaven. Some walk the earth." Reality is a mixed kind-of-thing to see reality as it really is we have to look, and to evaluate any form of legislation the legislation has to be written and only then can it be analyzed. Until then we are asking "How many angels can dance on the head of a pin?" And such conversations are pure motivated reasoning nonsense where people talk past each other if your goal is to win these conversations.

There is no winning or losing in these conversations instead it is an exchange of values where two people try to synchronize or they do not try to synchronize. What is the German Term again? It is often translated as "Transvaluation of Values," or a Re-examining which values you prioritizes and whether these values are valuable.