[wudangmonk]

There is a lot of these "dark ages were not dark" arguments but one can't deny that there is a difference in the world after the fall of the Roman empire.

The author emphasizes people drifting apart and cultural fusions, the natural loss of arts and books due to the medium on which they were written. All of this happens after the fall of any civilization.

Same thing could be said about the Aztecs or any other civilization, their people drifted apart, there was a fusion between cultures and their arts and books were lost due to the medium used to create it (it wasn't fireproof).

The empire didn't fall to any single external force but that is the nature of an empire, unless its conquered by an even stronger empire it always falls from within.