Doesn't need to be "we". Biden can say, "I am proud of my administration's sweeping accomplishments but my health has recently deteriorated unexpectedly and I therefore think it would be best for the country if I do not run for a second term."
I'm not here saying it wouldn't be messy and there's no chance of a huge drag on the new ticket that doesn't show up in polling now. I just think there is a good chance Biden is so impaired right now he's not capable of campaigning aggressively and he'll only get worse between now and November