Because Romans complaining about how decadent the current generations are, and how they were weaker than their ancestors, goes back at least as far as Roman literature does, a generation or two before the Battle of Zama and the end of the Second Punic War? If Rome was truly decedent then, they had another 350 years of growth in power and magnificence until their actual zenith. This suggests that the two factors were completely unrelated: Rome rose and fell for reasons totally unrelated to it's supposed cultural decadence, the toughness of its people, etc.

(Cue Peter Heather's argument that the fall of Rome was actually related to its inability to attract immigrants any more: for generations Rome survived and triumphed over other cultures because of its unparalleled ability to make new Romans- from other people's in Italy, from Gaul, from Egypt, from Germans, they could make everyone's sons into real, classically educated Romans, and that meant that their state and armies were sufficient. Then, for various reasons that he describes in his 2005 book _The Fall of the Roman Empire_, they lost that ability, and that was what doomed the Western Roman Empire.)