LBJ bailed on March 31, 1968.
There are plenty of people who could mount a three month campaign for the nomination.
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.