You’re right, but I think it was a combo.
October 7th spilled a bunch of gasoline on the ground. (Almost immediately after that day his polling trailed Trump’s.)
His debate performance dropped a lit cigarette.
In my opinion, you really needed both of those things for him to drop out. A physically struggling Biden that’s polling at 60% would’ve stayed in the race. A Biden with an excellent debate performance that was polling at 45% would’ve stayed in the race.
EDIT: typo
Smallville very directly addressed this in an episode where Clark Kent’s mom got mad at him for hiding an undocumented immigrant on their farm. He had to remind her his own paperwork was also less than legitimate.