Lots I don't know about the inner workings of American politics, but from my viewpoint:
Trump and the Russian bots want to push the "they'll cheat Bernie out of it" narrative on Twitter, and they created the Biden scandal around impeachment, which seems to have legitimately hurt Biden at the start of the primary push. This tells me they fear Biden as the opponent a lot more than Sanders, so that leaves me wanting to root for Biden.
As has been said, a Biden-Trump election focusses on Trump and what America's soul wants to be. Not sure that results in a Biden win, but it will certainly crystallize what America really is (if 2016 didn't already open eyes as to just how misogynistic and racist and old-boy establishment capitalist it truly is). A Sanders-Trump campaign is much more about capitalism vs socialism, and the US has been pretty well indoctrinated to hate socialism as Evil Empire old state Russia and Cuba communism. Vastly overblown, but simple fear tactics are remarkably effective in the USA in swaying the general populace.
Don't agree with those who say that the safe Biden choice guarantees a loss where we need a radical Bernie shift to energize progressives. Young people flock to Bernie, and that may be the future at some point, but young people for all their (some will call it "woke") activism don't for the most part follow that up with actually getting out and voting. Older folks get out and vote, which is why the old white men so often are left standing. Dems need women and the African-American vote, and Biden carries that.
Hopefully Bernie supporters won't be butt-hurt sore if he loses again and refuse to vote. They cried foul because Hillary took all the superdelegates even if Bernie led the polls, but the simple fact is that she still beat him 34-23 in primary contests, had a 450 delegate lead if you remove the superdelegates, and won the popular vote by 3.7 million in their run-off. Bernie backers need to get over the "he got cheated last time, and they'll screw him over again this time" mantra, he lost no matter how you look at it. He has created a movement, but right now America does not need radical to fight radical. Biden keeps pushing the "re-claim America's soul" narrative and it can work. Let AOC lead the march towards and more social democracy when she has a few more years under her belt - America could much more easily buy into her campaign than Bernie's IMO if that is the future. As for dirt, all Trump really can play up with Biden is Burisma and covering for his offspring who got a cushy job because of who his dad is, and do Republicans really want to push that narrative in a one-on-one fight, given what is happening with Ivanka, Jared, Don Jr. and Eric? Biden could easily say, "OK, let's open up that can of worms, but for both candidates - show us all how clean your familial dealings through the Presidency have been" and Trump can't refuse to play along but continue to push for transparency from Joe. With Bernie, it will be "socialism will destroy everything that America is" and Repubs are remarkable - I won't call them patriots - but flag-wavers for sure, who chafe at the idea that anything star spangled red, white and blue is less than the most awesome thing ever.
Also, Obama will heartily endorse his former VP as the Dem pick, and it won't feel fake. Same with the Clintons. Dems need a recovery election right now (a course correction from Trumpism rather than a true "left vs right" debate) and Biden can offer that safety. But I agree that his running mate needs to be the spark for progressive change for the future. Warren might fit the bill nicely there.