They see Biden as more of the same. They might feel the chance of a progressive in the white house could be bigger against 8 years of Trump than trying against Biden in 4 years.
If you not going to talk to their issues, expect 4 more years of Trump.
I just find that lunacy, because Biden may not be as progressive as Bernie, but he is closer to a progressive than Trump is.
Anyway, I'll let the Democrat voters do what they want to do - I'm closer to a Republican myself. But if I were a Democrat, I'd feel outraged that fellow Democrats would rather have Trump win than Biden.
EDIT: If anything, I'd see it as dangerous to allow Trump to win a second term. Because it reinforces his legitimacy.
When Trump ran, nobody believed he would win. They thought he was a buffoon (to be fair, he is a buffoon). But he won anyway.
If he wins a second term, it sends the message that American voters could support his style of leadership. That means that it will shift the tone of the debate of the 2024 presidential election.
If Trump had not won in 2016, I think US politics would be further to the Left than they currently are.