Newsom recently stated unequivocally that he was NOT running. So...
It's actually the perfect set up:
Take Biden right up to the wire - he croaks, steps down, falls over, whatever excuse.
OH NO! what are going to do???
Hey Gavin, I know it's a big imposition, but could you step in for poor Mr Biden?
Why Yes - I would be honored to serve the people!
No debates, no months of media coverage, questions, rallies - anything. Just step right in at the last minute and let zuckerbucks/dominion do the work.