In response to your question, De ... I think you would have to go back to the 1800s to find that situation.
LBJ saw the handwriting on the wall, himself, and decided not to pursue a second full term, and as you already mentioned, Nixon, like LBJ, recognized the inevitable, and decided to resign.
On top of his cognitive depredations, Biden is a bitter clinger (as is his grasping wife), so I'm not sure what it will take to get them to see the light of the oncoming train and leave "voluntarily." This could get really, really messy. It will be interesting to see how long the Bidens can ...