If it is a given that Biden won't have enough gray matter left to campaign in 2024, much less win, Biden will either complete his first term and not run again, or step down during his first term, allowing Harris to become President. In order to guarantee Harris a Presidency instead of throwing her to the sharks in a 2024 Democrat primary against better candidates, Biden must step down during his first term.
As to when Biden will step down, there are at least two factors to consider. When will Harris be polished enough to act even remotely Presidential and what is the shortest amount of time Harris can be in office as President without making it seem her Presidency was as affirmative action motivated as was her Vice-Presidency.
Harris could be gifted her Presidency around the same time Covid hit big during Trump's Presidency, early fourth year. Of course, there are so many variables and tricks up elite's sleeves that what actually will happen may be as unexpected and surreal to us as Covid was.