2021-2025: Joe Biden
2025-2033: Kamala Harris
Normally I'm hesitant to endorse the idea of 12 consecutive years of any party in the White House, but if Biden really only does 1 term, and 2024's nominee is a Strong Candidate™ that wins in their own right
(whether it be Kamala or not), the chances of a 12 year Democratic executive branch is better than it usually is. It's just easier for an incumbent to win, and a fresh candidate from 2024-onwards will be able to shed any negative issues Biden gets from COVID/the recession, while being able to run on the recovery.
All of this is made easier with the fact that Democrats simply have more voters than Republicans, and growing by the day with the ascension of the Millennial/Zoomer generations.