It never occurred to me that I might take the time to comment on a Substack article, let alone a Barro article (I'm a fan), but this analysis is gobsmacking.
Any voluntary change in the 2024 ticket is a giant red "salience" button for the issue of whether Biden is the right candidate. Further, as in every election, the incumbent's ticket is a referendum on the performance of the incumbent themselves.
Biden's biggest problem is that he's one of the most effective Democratic presidents in the last 50 years and nobody knows it. His biggest challenge will be to focus 2024 on his own accomplishments (and to resist being sucked into the black hole of negative campaigning that a Trump candidacy will generate). There's nothing Biden could do that would be more disruptive to that goal than to be the first incumbent since FDR to voluntarily switch running mates.