One thing that nobody mentions in the Medicare-for-All discussion, is that Medicare as it now exists is not exactly single-payer! 29% of people signing up for Medicare select a private Advantage plan. 17% buy private MediGap insurance. 15% have Medicaid. 33% have employer or military supplemental coverage. (numbers don't total 100% because some are older than others.)
That's about 90% that have something beyond true single-payer government Medicare! So I think the private insurance companies would still be doing that even with Medicare-for-All. Does Bernie not know this?
I think you have hit on the central point that is being overlooked. 64% of the American population already has some version of "socialized medicine" - single payer, like Medicare, Medicaid, FEHB, Tricare, etc.; or government provided, like the VA - so we're really only talking about a third of the population (plus the uninsured). I think it would be possible to do some kind of Medicare-for-all" system to get us all the way there - although it would not be as generous as Bernie supposes.
I think the balance is in a "public option"; government subsidies for the less-than-well-off; and allowing Medigap/Medicare Advantage style products from the marketplace to supplement for others. What is most important for success, I think, is to change the system without people realizing how much has changed. The Public Option will, in the long run, supplant most existing policies - including employer-sponsored insurance - but it can be sold, now, as bringing more people into the "market", and providing "competition".
Medicare is incredibly popular, as the ACA is becoming. The longer people have to get used to what they have, the more they like it - which Obama recognized and why the GOP could never kill it. FDR remained popular for more than a decade and affected politics for most of the 20th Century. Obama's post-presidential popularity continues to grow, I think for the same reason.