Yeah that's what I was thinking too. I'm going to re-read Canticle soon-ish but I wonder to what extent Miller endorses even the latter. I feel like his concerns were much more immediately spiritual than cultural. I'm thinking particularly of the scene where the main character of whichever volume meets the Pope up-close and is taken aback to see that his garments are moth-eaten and threadbare.
With respect to the ‘Fall of Rome/Dark Ages’ narrative I had Pico della Mirandola and Edward Gibbon in mind.
Considering their takes on monasticism, I suppose monasticism as the savior of civilization was actually a much later add on.