The only reason they chose Joe at the beginning of the primary campaigning is because they will do it all again. They are now stuck with him, because if they pull him before the convention, it is past the deadline to put a replacement on the ballot in three critical states.
If he steps down after the convention, Kamala would be the next president IF Biden wins. But that would result in a big loss of voters in swing states, making it a challenge to stuff the drop off ballot boxes.
The best the Dems can do is keep Biden on the ballot through the election. If Trump wins, count on Biden to step down and Kamala to pardon Joe and all the other Biden.
Also, taking Joe off the ballot before the election implies he needs to resign as president. That creates a huge mess for the Dems.
