Caren, it's folks like you who would hand the 2024 election to Trump.
Since Truman/Dewey in 1948, there have been 19 presidential elections. Incumbents --- including Truman and Johnson, each of whom became president upon the death of their predecessor --- have won 8 of those elections and lost 4.
The other 7 elections have been "open" elections, where neither candidate was an incumbent. Republicans have won 5 of those elections, and Democrats have won 2.
So the historical record is clear: incumbents usually win. Not always, but usually. And in elections where neither candidate is an incumbent, Republicans usually take the White House.
Obviously, historical comparison is not precise because things change over time: the electorate is different, the issues are different, and the Republican party has become a toxic force in American politics. But something else has also changed: people are living longer, healthier, and more productive lives in their 70's, 80's, and 90's. The fact that Henry Kissinger could go to China at the age of 100 is a testament to good genes and an apparently reasonably healthy lifestyle. But it's also a testament to modern medicine, which is keeping people alive and productive longer than in the past.
I agree that there are compromised seniors --- McConnell, Feinstein, etc. --- who ought to hang it up. But because of his unique position, Biden is not one of them. He is an incumbent, he has not exhibited the same health issues as McConnell or Feinstein, and he has shown that he can beat Trump. The stakes are just too high for the Democratic party to take a chance on a non-incumbant.
I'd rather take my chances with a Democrat with a good record, regardless of his age. I don't care if they have to prop him up to govern from a wheelchair: he's still far, far better than Trump. And if Harris becomes president, so be it. She's also far better than Trump, a terrible man who wants to gut our democracy for his own selfish purposes.