How is that a valid question though? That is the question you ask during a primary or in a contested convention, since the primary is over, and there are no plans for a contested convention the question is moot.
Like if this were a valid question the people discussing it would probably be pointing out that Harris is the most likely theoretical candidate that would win a contested convention, there wouldn't be so much time dedicated to who would best win in November, it's who'd win the contested convection.
We haven't yet nominated a candidate, and Joe's debate performance makes it a valid question. He absolutely controls the future of the party and it's candidate, and the only way there's a new candidate is if he names a new ticket and they go forwards into the convention with his blessing. His call.