Lbj dropped out and then his VP was trounced by Nixon. If Biden dropped out that late, Harris would really be the only option for reasons of incumbency and racial pecking orders and would probably get destroyed by Trump or anyone else.
Harris is their worst case scenario with Biden being the next worst. They can’t really extricate themselves from both of them, so Biden it will be unless he dies.
“his VP was trounced by Nixon”
Nixon trounced McGovern in 1972, but the 1968 result was closer. Nixon had about half a million more popular votes than Humphrey, although he did win in the Electoral College 301 to 191. (Wallace had 46 electoral votes.)