“No one has been officially nominated yet, so why would it be ‘extraordinarily difficult’ to replace FJB?”
Because the primaries already happened AND, more importantly, the filing deadlines have passed in every state.
It’s a real possibility that one or more swing states will not allow a Democrat nominee other than Biden to be on the ballot. Trump already has an advantage to win, this would add to that advantage.
Changing nominees at this point is the Democrats saying “do-over” but not really doing a do-over because they can’t redo the primaries and give people a chance to vote again on their preferred candidate.
All 50 states have laws for how political parties can select a candidate and get his or her name on the official ballot of the federal election for president. They can’t simply demand that these laws be ignored because they are inconvenient.
No state will deny a Democratic nominee a position on the ballot if they are chosen by their upcoming convention.
It just will not happen in the real world.
There is no Democratic nominee right now. There is nobody to “replace”.
All 50 states had laws on the books about mail-in ballots but they ignored the heck out of them when it was prudent for THEM. That said, I agree with your conclusion.