Biden absolutely fumbled this by even bothering to run. If there had been an open primary there would be some actual momentum behind the winner (which I doubt would’ve been Harris).
I don’t care to speculate her chances in a primary. If she had won it then she’d have a campaign far more likely to energize the Democratic base. If someone else had won it then they would have a similar advantage. Not having it, and finding out in the worst way that we should have used the fucking 25th amendment is the original sin of this campaign.