One nightmare scenario is that the Democrats decide to pull Biden off the ticket after early voting has begun in October:
https://www.ncsl.org/research/elections-and-campaigns/early-voting-in-state-elections.aspx
Then the Democrats will start crying that Democrats who had early votes for Biden have been disenfranchised, and they litigate state by state...
In the end they would lose in almost all state courts, but the litigation could take many months...
What does it gain anyone?
Over 200 years of everyone going to a polling place on the same day and now everyone has to have their own special voting.