Health insurance in the United States is a shit show. The Medicare for all that Bernie is proposing is a reaction to the inability of Congress to meaningfully fix the issue. I personally would prefer a market based approach to fixing the issues with obtaining medical care, but Congress can't do anything because there's just too much money and influence involved.
Just that it's in addition to the single payer system, which means the private one is cheap because the competition is 'free'.
As a % of GDP, Italy spends a lot less than the US in health care, and people live long and productive lives.
I mention Italy because 1) I lived there and 2) it's not some perfectly run place where everything works. It's not, and yet the health care is still better, overall, than in the US.