It seems clear to me they have to nominate somebody else, since this wasn't a one-off affair of a person failing to perform in a debate. Trump offered many points of attack which a politician with Biden's experience should've been able to exploit. For instance, Trump seemingly admitted to Putin having confided in him that he was going to attack Ukraine beforehand. Pretty drastic stuff.
That Biden wasn't able to pick up like thirty opportunities offered on a silver platter says more about his state than a shaky hand does.
I do wonder what the damage of pulling Biden as opposed to letting him run would be for the Dems, though. I mean, in America's polarised society there'll be people who would've voted for Biden no matter what. But what does it say about the Democratic Party if Biden has to pull out and thereby basically say: "Yep, you were always right, I'm too old"? Would that not be damaging to Newsom or every other candidate as well?
Or would the Dems just be like, well, after all we haven't formally endorsed Biden as our candidate yet, so …?
How do you justify pulling out at this stage without a sudden cancer diagnosis or something like that?
By the way, would it be constitutionally permissible for Biden to say, I'll run, and if elected I retire half-way through the term and my running mate takes over the presidency? Mind you, I'm not asking if it would be popular, just if it's legal.