I think a lot of the opportunity cost of there being one & only one interesting city. It took generations, but Rome's fall lead to other places being able to find their own identity & value. Civilizations value became decentralized.

This is also a fine example of the classic Trimurti, the cycle of creation, maintaining, & collapse that defines existence. Rome flourished & became, but the world had been "stuck" with Rome for so long, had maintained it for so long, & it's presence was not leaving room for other acts of creation. This article largely asserts that destruction was necessary: Rome was facing a millennia+ old version of "The End of History" hypothesis. Which, to double back to the Trimurti, is against the cycle of creation & existence.