By this point, this is far from an original thought. But I have concluded Biden should withdraw from the presidential race, even if - as is likely - that means Kamala Harris takes his place on the ticket. If Biden remains the candidate, I still will likely vote for him, because he's a lesser evil than Trump. But there are both electoral and governance reasons for him to withdraw.
Electoral reasons:
Polls have long shown that Biden's age is an issue for voters, and it's gotten worse since the debate.
Probably no more than 2-3% of the electorate will switch because of this issue. But that could easily be decisive in a close election!
Kamala Harris right now scores the same or slightly better than Biden. But as the nominee, she would get a boost. If Biden resigns the presidency and she becomes president, the boost will be greater. We're talking only slight boosts (a couple points). But that could easily be decisive.
Biden could easily have more "senior moments" between now and November. That would make things worse. It isn't guaranteed he will, but it certainly could happen.
If Biden stays in, there will be a lot of attention focused on him and his age/health. With him out, more attention can focus on Trump and his awfulness (including his own cognitive issues).
Governance Reasons:
If Biden is in cognitive decline, it could easily get worse over time, and likely will, given his age (he will be 86 by the end of a second term).
Even if his current condition is OK, he could easily get worse in the next 4 years, and likely will. A president in his mid-80s is inherently risky in this regard. Maybe the risk would be worth it if Biden were an irreplaceably great leader, like Gladstone, Adenauer, or Churchill (they all served as head of government into their 80s). But Biden is nowhere near that level.
I'm no great fan of Kamala Harris. She's not who I would choose out of the current set of prominent D politicians. But at this point, her abilities over the next 4 years are likely to be far better than Biden's. And, like him, she's a lesser evil than Trump.
The presidency is a high-stress job where you have to handle sudden crises. That makes the danger of cognitive decline worse.
Counterarguments and my Responses
Criticism of Biden is "ageism:" Like it or not, age has an impact on cognition. That makes it very different from race or sex, which in most cases are irrelevant to leadership and cognitive performance.
The primary electorate picked Biden and it would be undemocratic to replace him: Democratic processes aren't always best. The primary system has many flaws, of which Trump is Exhibit A! Plus, his cognitive flaws are more evident now than during primary season. In the long run, I would probably abolish major-party primaries and go back to the old convention/smoke-filled room system (perhaps sans smoke). I voted for Biden in the 2020 D primary (he was far preferable to Comrade Bernie Sanders). But he should never have been the candidate this year, given his age.
He did a great job so far: I think his work so far is a mixed bag. But, in any event, this argument ignores the progressive nature of cognitive deterioration. You might as well argue that Michael Jordan would be a great NBA player now, because look how great he was when he was younger! And Biden was never as great a political leader as Jordan was a basketball player.
There will be chaos, if he gets replaced: Once the Dems settle on a new candidate, they will be unified again, and that could happen quickly. At any event, it's a lower risk than keeping Biden on the ticket.
Other presidents with bad debate performances (Obama), or concerns about age (Reagan) won reelection: Those guys had substantial leads over their challengers, and much higher approval ratings than Biden. Plus, Reagan in 1984 (When he was 73) was a spring chicken compared to Biden (who will soon be 82).
It's unfair to push Biden out, without demanding Trump drop out as well: I agree Trump is awful and should drop out! He should be in prison. But Trump's awfulness actually strengthens the case for removing Biden, because a Trump victory would be far worse than one by a conventional political opponent. It may be unfair if Biden gets forced out, but Trump doesn't. But it would be even more unfair if Biden's obstinacy leads to a Trump victory. What's good for the nation and the world trumps (pun intended!) fairness to an over-the-hill politician.
If Biden decisively shows over the next few weeks that the debate was an aberration, and climbs in the polls, I will admit I was wrong. But I don't think that's the most likely scenario.