Biden has no intrinsic momentum or popularity. He is not an insurgent, is not (falsely) promising radical change. There is nothing there to appeal to the masses or evade conventional polling. A replacement candidate suffers two inherent handicaps: they are compromised, by coming from a split party that has no time to coalesce around their candidacy. Sure, the absurd US primary system means that voters are used to sworn enemies suddenly becoming best buddies (and VP picks), but not so close to the date, with all the bitter soundbites fresh for campaign weaponisation.

But the whole Democrat organisation is compromised as well: Trump's line will (quite plausibly) be: "These guys told you yesterday Biden was fine, that he was their man, a great leader. Now they say he's senile! You can't trust 'em. All lies! Look at me - strong, virile, golf handicap of -3 and capable of recognising a picture of an elephant! Bullet-proof!"

A campaign by Harris or whoever else would start out from such a poor position. of mistrust, unpopularity, confusion, lack of voter recognition. I can't see it happening. Unless they pick Clooney!