I think the missing factor to consider is that Biden is unpopular mostly because of the media telling absolute dogshit lies about him like that he was bad for the economy.
Sure, he's old. But if "elderly" was a disqualifying factor, then "insurrectionist" would be, also. I think the issue is that the media is going in the tank against the progressive, as they tend to do, and that that has the potential to swing against any genuinely decent candidate that the Democrats want to put forward. I sorta hesitate to say that a good answer to that is "okay well let's just put forward a more corporate friendly candidate who will be 'popular' with the people whose perception is shaped by the media."