This is interesting. I'm a bit of a Watergate buff, and one of the key elements of Nixon's resignation was his staff's insistence that resigning never be suggested to him directly as a course of action because of fears about doing so being perceived by history as a coup d'etat. It was very important that Nixon alone make the decision to resign. You could provide him with information of his support going to zero, but not offer resignation as "the only" possible course of action he could take in response.
I think something similar has happened here. I do think up until the debate there was likely little thought of Biden stepping down, but a smart group of staff (Biden's or Harris's) would have been making contingency plans as they watched the President slowly lose his faculties. But I feel confident that when Bob Woodward writes this book, we'll find the same dynamic whereby the staff was planning, and providing Biden with data on public perception, but never suggested directly that stepping aside was the best or only tack to take. Like Nixon I believe Biden firmly felt he could continue in the role up until just before he realized what was best for the nation.