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.
I'd bet anything there would be at least one narrative a bunch of outlets and pundits go with that keeps "Biden too old": they'll just claim he was "too old" when he picked Harris as VP and now that choice just has to be retroactively invalidated.