I never denied that there was broad support for 'universal coverage'. I even pointed out that people want improvements in my previous points.
The problem is, there is more than one way to implement 'universal coverage', and Sanders has picked a method that does not have majority support.
What YOU think is best for America is not as important as what the average voter and/or swing voters think.
Sander's 'medicare for all (no private coverage)' plan is only supported by 13% of voters. More than twice as many (32%) may want universal coverage, but they also want private options. There is no guarantee that any of these 'want both public and private options' will go for Sanders if they are given a choice.
And remember, in another poll, over 2/3rds of people rated the coverage and quality of their own personal health care coverage as 'good' or 'excellent'. If you go to those 32% of people who want universal coverage but with private options and say: We want to take away something that you currently see as "good or excellent", how many of those people will be on board?
https://news.gallup.com/poll/245195/americans-rate-healthcare-quite-positively.aspx
Many voters may not like what Trump is doing to the ACA, but if they are given a personal choice that they may think will harm them personally, they may end up rejecting Sanders and his plan.
Now, you MIGHT try to claim "It'll be cheaper", or "Other countries do it" (which would lead to a full debate on its own), but unless Sanders has some sort of magic wand that will make people automatically dismiss any personal opinions or preferences they have "for the greater good" then whatever supposed benefits his plan have are moot.
Sigh.... yes, he polls well. The candidate who went to a 'death to America' rally (which hasn't been brought up yet in any campaign ads) is polling well. And hey, maybe the republicans will be nice and not bring up his 'death to America' rally. After all, the Republicans don't believe in playing dirty, do they?