But why does the concept of sole authorship matter? It is an essentially 19th century concept grown out of the bourgeois notion of property and it is alien to the times when the said works were actually created. None of the "great masters" was the sole authors of the works attributed to them - these were the collective products of the workshops owned by these masters.
This whole debate reminds me of the efforts of xtian and bourgeois moralists to support their bullshit moral tales by "finding" embodiments of them the nature. Likewise, the bourgeois ideology depends on the notion of individual ownership and is trying to project that notion on everything it manages to put its greedy paws, especially works of art.
Wojtek