I strongly disagree.
First, the fact that Trump is the president makes him a legitimate candidate. Pretending he can't win helped get us into this mess. If anything, Dems need to work harder at combating the notion that he's a guaranteed one-termer. Trump can win. It's even likely that he will win, with all the cheating he will do. Dems need to know that they have to rise up and make something happen.
Second, you're arguing from the losing framework that has been behind 30 years of Dem failure. It's not about bringing in new voters from the left (although energizing the base is important). It's about persuading people who aren't ideologically fixed to jump aboard. And there are more people like that than you realize.
There's a reason so many voters had Bernie and Trump as their favorites in 2016 even though they are far apart on the ideological spectrum. Many if not most people are just looking to be led towards a strong vision that will help them and their families. If the only option for a strong vision is negative, like Trump, many will gravitate towards it, and those who can't stomach it will sit out the election. If a strong positive vision emerges from the left, it will take people the other way. Most people don't really give a shit about the details of something like MFA. They just want their shit to be covered without paying more, and the polling merely shows where they think that will come from at the moment. If the Dems get strongly and confidently behind something like Bernie's vision, you'll move those polls. It's already happened to a substantial degree because of Bernie, which is why this policy discussion is even happening right now.
Thinking that strong progressive ideas will scare off voters is a self-fulfilling prophecy. Following polls instead of moving polls may eke out one election and get rid of Trump. And that would be better than losing, certainly. But it won't get rid of Trumpism or the reasons why Trump won to begin with. And we'll be right back here in a few years with some asshole Trump disciple who is smart enough to get more shit actually done.
If Dems focus on people who right now would rather vote for Donald fucking Trump than a candidate who is for universal healthcare, student loan forgiveness, some version of free college and skills training, universal child care and pre-K, and huge government investments in green technologies, then we've already lost.