Joseph Smith said many papyri were bought, and only some fragments survive. Could the Book of Abraham have come from a scroll that was lost, so the surviving funerary texts are beside the point?

This is the church’s own preferred reply, and it is not unreasonable. Some papyri were lost, and no one can translate what no longer exists. Two things keep it from settling the matter. First, Joseph Smith’s explanations of the surviving facsimiles, printed with the Book of Abraham, assign meanings that Egyptologists say the images do not carry, and those facsimiles do survive. Second, the missing scroll reply moves the claim beyond checking, which is a fair thing to notice about any argument. The honest summary is that the surviving evidence does not support the traditional account, and that account now rests on materials no one can examine.