Three incumbent Presidents in my lifetime decided not to run for another term, and they did so because they couldn't get reelected, however much Biden may be praised for doing the patriotic thing, which was also the truth. Harry Truman said he wouldn't run in 1952 because he couldn't get a good enough deal to end the Korean War and because McCarthyite accusations against him had hurt him. Eisenhower had a clean slate. He took the available deal on Korea and bided his time to finish off McCarthy. LBJ declined to run because he could not get a negotiated settlement with North Vietnam and because Eugene McCarthy had nearly beaten LBJ in the New Hampshire Democratic Primary. Biden had to resign the nomination even though the polls with Trump remained close because he was told the polls were bad in the swing states and that the Democratic donors had dried up.
Moreover, there is plenty of time to shift to Kamala as leader. Remember that the entire British election season lasts just six weeks from the time the election is called until it is decided and Kamala will continue Biden's policies both foreign and domestic and can face Trump on the issues of abortion,and Ukraine in a lively manner, asking Trump in a debate why Trump never criticizes Putin and that Trump is responsible for overturning Roe v. Wade.
Another thought. Harris' vp nominee will have to be a white male so as to balance the ticket. even though the obvious choice would be someone from a battleground state, which means Wisconsin , Michigan or Pennsylvania. Gov. Evers of Wisconsin is just not mentioned as a heavy hitter. That leaves out Gov. Whitmer of Michigan and Sen. Klobuchar of Minnisota, but not Gov. Newsom of California. He is a heavy hitter but might be willing to take the job because, as the expression goes, it puts him one heartbeat away from the Presidency. LBJ made the same decision and not just to get Texas' electoral votes, though Sam Rayburn mentioned it as did JFK. The trouble with Gov. Shapiro of Pennsylvania is that it would make the Harris-Shapiro ticket too Jewish. Under Kamala, the spouse of the President would be Jewish. Harris's step-daughters refer to her as "mamala". That nearly happened once before because the wife of Michael Dukakis was Jewish and people asked her if she would put up the White House Christmas tree and she said she would. Harris's step-daughters refer to her as "mamala". So no Shapiro as vp. Sen. Mark Kelley of Arizona seems a good choice.