At one point in the Cur Deus Homo S. Anselm suggests that a reason why man was redeemed was in order to make up for the number of angels who fell. The beauty of the Celestial City demands a determinate number citizens; since some fell others had to take their place. This argument always seemed odd to me, but now I think that I can say more determinately why it is odd. There is indeed a truth that he is pointing to – our salvation is for the sake of the beauty of the Celestial Jerusalem – but he is looking at things in his usual back to front, upside down way. It is a way of looking at things that doesn’t do justice to the “rhythm of increase” in the manifestation of God’s glory. For why did God permit Lucifer and his angels to fall in the first place? Surely for the sake of the greater glory that was to come from the Incarnation. In other words one can turn what Anselm is saying around: the reason why angels were allowed to fall, and to tempt Adam to fall after them, was that God would have occasion to become man.
What comes last in time is often what comes first in the order of final causality. Everything that God does ad extra is ordered to Christ as its Final Cause: not just that he permits the original order of nature to be destroyed by sin, but that he created that order in the first place.