[I]t requires the existence of a copyist prepared to make clarifying interpolations, who thinks this clarification is needed, who actually made this interpolation, at a point where they could influence the manuscript tradition, and who managed to influence it so much it wound up in all extant manuscripts. That’s a figure with really quite specific properties we have to propose to explain this one verse.
Why is the existence of this hypothetical copyist less problematic than the existence of the historical Jesus? The historical Jesus can explain a lot more than a single verse in Paul, so is vastly more explanatory and parsimonious than a copyist with very specific properties the existence of whom is only proposed to problematize the existence of said historical Jesus, by calling into doubt a verse which probably isn’t even an interpolation.
This seems to me to get at a key point – genuine skepticism, of the sort that scholars aim for, has to look with the same skepticism at the claims being made for one’s own position, as for those one is arguing against. Asking whether the text is likely to reflect what was originally written is appropriate – but so too is asking about the likelihood of interpolation.