I ended up getting into a prolonged discussion about Biden a while back, with interesting points on either sides. In terms of a short term move, Biden made a lot of sense. They lost in 2016 because they ran a woman for President, who was married to probably the most successful Democratic President and followed on from 8 years of a black president. Trump could whip people up into an anti-woman, anti- black, anti- sleeze (Clinton) block.
That's just a lot harder to do with an old, white, fairly moderate/conservative man. The attacks on Kamala Harris have failed. The attacks of saying he's a pupped for the left have failed. I'm not sure what else Trump has. I know it sounds harsh, but he's not black, he's not a woman and he's from the South of America, so for middle America they are just not angry towards him.
I would have gone Sanders, and I still think Sanders wins, but probably has to do so in a different and riskier way. Trump would at least have more amunition against him. He has nothing against Biden. He calls him old, which has stuck, but I'm not sure that really angers people in the same way.
Biden will face a medium term challenge though. He is going to be flanked by the left and the right in America and put under enormous pressure from day 1. There's no time for complacency with the Democrats. Unless there is major reform, there is a big medium term risk of the far right re-emerging.