If you have a medicare option on the marketplaces, then people are free to switch to it of their own free will, rather than being forced off of their existing employer-provided plans.

If we are right that Medicare for All is better at providing care at a lower cost, then people will naturally want to move into it. Over time, fewer and fewer people remain in the private market because of how much better the Medicare option is. Eventually, the vast majority of Americans are all included in this pool (because who wants to pay more for worse care?) and it will result in a very natural path to true single payer.

This is better than M4A for a couple of reasons. First, political. M4A absolutely will not pass the Senate, even if you get rid of the filibuster. There simply aren't enough votes when Joe Manchin and Kyrsten Sinema are the deciders. Secondly, M4A would be a huge shock to the system, which will cause an insane amount of anxiety. Forcibly kicking 150M+ Americans off of their employer-provided health insurance plans sounds like absolute chaos. Remember how badly "if you like your doctor you can keep him" played out? This would be a million times worse. Not to mention the short term damage it would do to our economy. Under Bernie's plan, we're talking about a net loss of millions of jobs. Think of all the people who work in health insurance and hospital billing. Most of those people would lose their jobs. And while new healthcare jobs (doctors, nurses, etc) would get created, those jobs require specialized schooling and there's already a shortage as is. I agree that in the long run a single payer system is better, but let's be honest: there will be short term pain. A Medicare Buy In allows the market to gradually adapt to the change through an organic process. In a nutshell, this plan gets us on the road to single payer without scaring the shit out of everyone.