I actually do think Biden can be a good president and not just a lesser-of-2-evils. Biden has good policy skills, he was the one who oversaw the stimulus package and directed it to renewable energy, he was the one who helped get the votes for ACA. He's got the skills you need for recovery.
Obviously a good chunk of the base is nominating Biden because they want Obama's third term and he's as close as they can get to it. If Biden were to win, that would be progressive because that would cement for good the transition of the Democrats to the Party of Obama. It would be the triumph of Obama's legacy over Bill Clinton's.