There are some logistical issues with changing candidates at the last minute, particularly with a Presidential ballot where fifty states have a complex variety of rules.
In addition, we are now in the age of early voting, so many Democrats will have already cast their ballots for Biden in October.
Check this out—scroll down for state by state details:
https://www.ncsl.org/research/elections-and-campaigns/early-voting-in-state-elections.aspx
This is why the best strategy for the Trump campaign is to hold their fire against Biden until well into October—and then blast away 24/7 with ads of every kind featuring “The Real Joe Biden”.
“There are some logistical issues with changing candidates at the last minute”
Which is why I believe the left are more concerned with the VP selection and plan on taking Biden out of the picture early, should he somehow win. Suddenly the entire left would be “woke” to Biden’s incompetence.