Gavin Newsom would drown his mother to be president so the fact that he’s not open to any ballot-overhaul schemes should tell you a lot about whether that has a ghost of a chance of making him president
How do you know he isn't? Anyone who might want to swoop in and get the nomination in a contested convention would publicly support Biden while demonstrating how they would be better at taking on Trump. Newsom has been doing exactly that.
Also given how pathetic the California legislature has been under his governance, I don't really know how far to the left of Biden he would be, either nominally or functionally.