If Hillary Clinton had won she might have gone on to be a good President and served two terms. But her defeat made her a two time loser (even though she won the popular vote easily, so kind of unfair) and not only that but a loser to as poor a candidate as Trump. Trump in 2016 wasn't the political heavyweight he is now, and his base was only starting to form, and I think most of his supporters didn't really believe he would deliver. And yet Hillary still lost.
Hillary's Clinton's deplorables comment was, in retrospect, a decisive moment. She was saying that American is a smart, good nation and you can ignore the dumb and bad voters and still win. Unfortunately, that turned out to be false.
She then made things worse in the aftermath by focusing on the injustice of her defeat rather than her own failings which may or may not have been fair but has the whiff of bad loser.
There is also the possibility that Bill Clinton's name will come up again in the Jeffrey Epstein investigations. There is currently a court case with Prince Andrew and more names may come out. I believe, as far as I know, that the claim that Bill has been on Epstein's jet and/or island is based on credible evidence. Search for "Jeffrey Epstein's former pilot testifies Bill Clinton, Donald Trump, Prince Andrew flew aboard Epstein's private plane". Totally unfair on Hillary, but a reality nontheless.
The democrats need to drop her.
She may have lost perhaps partly because people didn't warm to her, and may be fundamentally unlikeable. Is there a bit of sexism in that? Maybe a tiny bit, but mostly it's on her. She also represents the elite at a time when the democrats need to connect with the working class.
I think Bernie Sanders is too old now.
Hopefully Biden can step aside if his approval rating and head to head vs Trump is low. However, it would be smart of him to keep cards close to his chest if he is planning on a single term. Because once it's known this is his only term, he starts to be considered lame duck. Perhaps announce suddenly in late 2023 if he isn't running.
Warren or Buttigieg are both candidates I'd be open to but there may even be a need for someone completely fresh. Ideally someone with a good career and political experience, but not someone who has been thought of a front runner before. Democrats need someone to connect with the working classes and take a centrist position on certain divisive issues, rather than dividing the country further.
However one way to look at it is just to put up whoever polls best against Trump. The bottom line is Trump must lose to protect American democracy.
I think the democrats should already be soft testing potential candidates in certain ways like town halls, speeches, TV debates and interviews so that they are not suddenly required to come up with a candidate if Biden dies, becomes ill, or decides not to run. Either that or Biden announces as I say late 2023 to give plenty of time.