The best argument anybody has at this point is that the media narrative itself is too damaging to keep him around, but that presupposees that the median narrative is related to a specific Biden weakness and not a structural problem with our media landscape.
Yes, if we swap out Harris the "he's too old" narrative will go away because he's not going to be the nominee anymore. But I guarantee you sure as shit that there isn't going to be a new damaging narrative, either specifically related to Harris or related to the process by which he was replaced.