A low birth rate due to extended periods of prosperity and low perceived external danger is the primary reason that Rome and most other powerful civilizations collapsed.
Rome had a very high birth rate when under extreme pressure from Carthage and a low birth rate a few generations after Carthage, their last serious threat, was finally destroyed.
Asimov’s Foundation series was a major influence on me.
That and the fall of Rome, from which Foundation takes inspiration, is what convinced me that we need to extend consciousness beyond Earth to avoid a potentially infinite dark age.