That's why I asked that if one intends to make such assertions regarding Sanders and the general election that they should provide evidence.
I had a longer post that was eaten by the void but it's easy enough for people to look for themselves at general election matchup polling for Trump vs Biden and Trump vs Sanders. You can even look at those matchups for individual states such as those which effectively won Trump the election in 2016, Wisconsin, Michigan, and a Pennsylvania.
https://www.realclearpolitics.com/epolls/latest_polls/general_election/
Is Biden currently still the best odds to beat Trump? Yes he is. Is Sanders currently still beating Trump both in general election and those states polling? Yes he is, and he's not too much lower than Biden.
Is the sky is falling attitude from the democratic establishment regarding a Sanders nomination silly? Yes it is.