Whether he’s up to the job or not (& I think he’s not) the narrative is there now and every single misstep he does will be amplified
He needs to be gone now, but theres huge issues with that, legally the campaign funds he’s raised cannot go to anyone else, except Harris & Republican lawmakers are already preparing appeals should to block her or anyone else getting them. A new candidate , other than Harris, would need to get the massive campaign team in place, book speaking venues etc etc in a few weeks.
Realistically Harris is the most likely replacement but shes not that popular and the Harris birther conspiracy theories are already out there & already being mentioned by Trump: very good wired podcast about them here:
https://pod.link/1516843565/episode/210b88c73fa5f17bafa21d5ee46edb3b
all that said in some polls shes beating Trump! and with Biden out of the way and a younger candidate on offer all of a sudden the focus would very quickly switch to Trumps declining mental state and some of the very senior moments he’s been having that are completely overshadowed by Biden
Another option and probably a long odds bet that might get you a good return is President Michelle Obama….