Yes, it's false to claim that current polls support Bernie's version of MFA. It's also false for Kamala to claim that her plan is supported by 200 economists who signed a letter in support of Bernie's MFA. Things like that should be corrected. But I don't really care about the current polling for single payer. I care about which candidate can persuade voters to give it a shot. Was the ACA a huge electoral success when it was enacted because it was net favorable in the polls? It quickly moved to strongly net unfavorable, and Dems lost the House and then the Senate and then the presidency, in no small part because Obamacare was hated by the public. And that was because Republican messaging moved the polls. You want to do that all over again.
Yes, the current center is being moved left. But that isn't because of centrists. It's because of the far left. The quick progress on gay rights is entirely because of what was called the "radical homosexual agenda" by many at the time. It happened in spite of centrists and the conventional wisdom, leaving people astonished at how something that helped Kerry lose in 2004 could flip so quickly after that. And the recent progress on health care is in no small part because Bernie moved the conversation far towards the left, making the center move with him. The ACA helped not by moving people to the left, but by having so many problems (and causing so many Rs to be elected that undermined it) that the need for something better is even more desperate. Pot legalization isn't liberal; it's libertarian, and the reason it's happening is because so many baby boomers did drugs that it's an acceptable corporate opportunity.
Hillary lost for many reasons. One of those reasons was that she did not inspire the left, and was hit for all of the shitty centrist policies of the 90s that she supported with Bill, as well as voting for the Iraq War. Superpredators. NAFTA. All of those things hurt her with a significant chunk of voters in 2016. More turnout from the left and working class would have beaten Trump, and centrism is one of many reasons why she didn't get enough.
What do you consider the top 3 Democratic policy achievements since Bill Clinton was elected? Getting the Heritage Foundation's health care plan passed and taking the blame for its failures seems the biggest to me. With successes like that, what's the downside to going big?