In the aftermath of the June 27 presidential debate...

Joe Biden was appallingly old and feeble on Thursday night, even after he warmed up. I think it's reasonable to doubt he's fit to be president for another four years, and many folks are clamoring for the Democrats to find another candidate. While I sympathize with their alarm, we can't.

See, the reason why the Republicans are the 'bad guys' in our current predicament is that they have no principles, only appetites. (viz Trump's presidency, the conduct of Clarence Thomas and the recent SCOTUS rulings, the clown show of the GOP controlled House, etc.) They want Mr. Trump in power however they can manage it, because nothing matters more than getting what they want, and the plan outlined in their Project 2025 will end our democracy,  which is why it's so important to stop them.

Being the 'good guys' (or less bad, anyway) means we have to accept limitations on what we can do and how we can do it. Legally, there's nothing to prevent the Democratic Party from nominating someone other than Joe Biden for the November election, but disregarding all the state primaries we have held (in which the president got more than 14 million votes) and installing someone else simply because we no longer fancy the winner's chances, makes a mockery of the very name of the Party, much less its principles. 

If Pres. Biden drops out, the Dems would obviously have to figure something out, but unless he does that, I don't see a legitimate basis for displacing him.

If he does resign, then I don't see how we can avoid VP Harris taking his place, for the same reason; while folks may not have given it much thought, she was nevertheless on those same ballots with Joe Biden, and she got all the same votes that he did. They're a joint ticket, so if he departs then she steps up. That's the how the actual offices work, and I don't see any ethical basis for upending the internal processes of the Democratic Party just because we're concerned she'll lose the election. That's the consequence of making her Vice President. (The only way around THAT would be for both of them to drop out, which is just madness.)

All of which is to say: Joe Biden & Kamala Harris are the Democratic nominees, and it's too late to change it.