Then all early ballots for Biden would need to be discarded. Were Clinton to replace Biden in October every ballot cast for Biden could not in any way be transferred to Clinton of whoever takes Biden’s place.
Also the dangers of early voting ought to be obvious to the Dems. Had there been early voting in 2000 there likely would have been no crisis. Bush was comfortably ahead until the DWI story broke a week before the election. His lead evaporated. With early voting following the polling then he wins. This is why October Surprises are now September surprises and the debates have lost their impact.
There is the argument that when the Party formally replaces a candidate whether that candidate withdrew voluntarily or for reasons of incapacity the Electors are free to vote for the Party's replacement. I.e., there is no "faithful elector" issue due to impossibility and, as a practical matter, who else are they going to vote for?
Which of the two possibilities sounds most reasonable?