They voted for the Biden/Harris ticket. So if Biden can’t do it, Harris gets to run for President. She’ll “finish” what Joe had started. There’s no justification to go with anybody else this late in the game. And like you said, it’s not Democratic.
It is an interesting “technicality” but in fact the Presidential nominee can dump the VP nominee as late as the convention—and in fact later than that if the party rules permit it.
Remember when McGovern dumped Eagleton?
At the end of the day the VP nominee is on the ticket at the pleasure of the Presidential nominee.