This is the best summary of all of this I've seen and the thorny questions ahead. It was FiveThirtyEight/Nate Silver's loss when they got rid of him.
I agree with him I think Biden will be the nominee at the convention. If Harris was the obvious alternative, people would have already coalesced around her, but that isn't happening as people do have doubts about how well she'd do (polling suggests she might have weaknesses in swing states/key demographics).
"So how did this become such a national conversation? In my view, left-leaning and centrist columnists and commentators from the Bulwark, CNN, MSNBC, the New York Times, Pod Save America and The Post were particularly important. They created a kind of feedback loop: These pundits voiced their concerns about Biden right after the debate; then Democratic politicians, donors and staffers contacted them anonymously and encouraged them to keep airing these concerns; then the pundits repeated the private comments of key party officials.