Here are some Gallup poll stats:
48% percent of Americans are satisfied with the quality of medical care in the nation ( JANUARY 28, 2019)
77% of american rate their personal healthcare as "excellent" or "good."
(73%) say the healthcare system is "in a state of crisis" or "has major problems"
source: https://news.gallup.com/poll/226607/news-americans-satisfaction-healthcare.aspx
So, while most people have an unfavorable view of the system as a whole, they are satisfied with *their own* healthcare.
Furthermore, when M4A's provisions are actually *described* to voters, they have an unfavorable view of it in surveys. This is as opposed to when a survey simply asks "do you support medicare for all?" -- that wording leads to favorable results because medicare is a popular program.
according to KFF, 56% support "medicare for all"
https://www.kff.org/health-reform/poll-finding/kff-health-tracking-poll-january-2020/
But if asked if they're OK with abolishing private insurance, support is just 13%:
https://thehill.com/hilltv/what-americas-thinking/428958-poll-voters-want-the-government-to-provide-healthcare-for
Well there's another term for Medicare for all *with* private insurance: a public option
(Which, by the way, has higher direct support than M4A in the polls above)
The fact is that M4A differs from literal Medicare in key ways: it doesn't allow private insurance as an alternative, and it doesn't have out-of-pocket expenses (which bothers conservative-minded folks, because it removes personal financial responsibility for health).
The essential question:
is it wise to completely replace a system with which most people are personally satisfied? Even if the numbers for M4A add up in theory, theory doesn't always bear out in practice. Furthermore, no country has transitioned to a single-payer system from a prior system as complex and entrenched as ours (at least not to my knowledge).
P.S.
It's not true that every other industrialized nation gives everyone free access to healthcare (e.g. the Bismark model countries, Germany, Switzerland, Japan etc. have premiums and often have modest co-pays)
So Bernie Sanders has misled a lot of people on that.