I don’t know who the answer is - my personal preference remains Bernie and I think he would be an easy sell
I have to disagree here. All Biden had to do was alleviate fears among independent voters about his age and health by simply not walking out there looking like a dementia patient that wandered off again. He failed at that, miserably, which only magnified and reinforced the exact fears among independent voters he was supposed to be alleviating. And, as expected after that disastrous performance, there were a lot of people waking up this morning saying "See, I told you. He's too old for the job."
Regardless of Sanders' mental fitness, health, competence, political positions, etc., replacing Biden on the ticket with somebody even older than he is would absolutely be the most tone-deaf thing they could do.
Policies aren't the problem. If these people cared about policy, they'd just stick with biden regardless of age.