My response to that would be that in 2020 he had the weakest campaign and still had the majority of voters rally behind him.
The person with the second most support in the party is just as old and is continuing to endorse Biden.
Anybody but Harris at the top of the ticket as a replacement means whoever gets the nomination will have a billion dollar deficit in running against Trump, meaning they have no shot at winning (seeing that they would be running against the only person in modern history who has won an election despite raising less money).
If Biden’s mind is as bad as we think it is, Kamala has been acting as president. She’s been a fairly good president, all things considered.
If Biden can’t win this, nobody in the roster can.