I think Gavin riding with a dramatic convention nomination, a few days after Biden addresses the nation LBJ-style, would be great TV/Social Media, would run his candidate ID up to where it needs to be, and would give all the folks bitching about having two old people to select from a real choice.
The right's greatest advantage against a Dem presidential nominee is the slow-drip-drip of turning people against them on their media outlets, in chain emails, and at the watercooler. It's what made Clinton, Bill a great candidate. "Who? Well, I gues I can vote for him." But then made Clinton, Hillary a terrible candidate. "Yeah, Billary. Man is that pants-suit lesbian actually be president. No way!"
Dems with little record of being slurred by right-wing media, and the often complicit mainstream media, fare MUCH better than the old hand.
Biden won because Trump fucked up COVID. Period. We probably could have, in retrospect, nominated a number of other folks and pulled out that win. Of course, who eles could've made it through the Dem nominating process is a totally different discussion.