wouldn't letting a single bishop's perception settle the matter be a papist idea?
Great question!
I don't think so, because at this time you still did not have a consolidation of authority and the power of the state to enforce it. Christian bodies were still somewhat free and if Athansius' position on the matter was controversial local and regional churches would have fought against it. As it was his position on the issue of the Canon was accepted because it was consistent with the understanding of Christians all the way back to the Apostolic Era.
I find it interesting that this was all done independent of any Bishop in Rome, or grand council.
Is it possible Rome did not want to take a solid position so they could change their position as their "traditions and doctrines" changed with the times?