Except the elders in the late Roman empire who complained about how citizens would rather cut off their thumbs than fight in the Roman army. Their decadence did actually doom them.

In general in history, decline and an increase in decadence is the norm, reductions in decadence are the exception. So many elders are in fact correct when they complain about the next generation. But the decline can often continue slowly for many generations before an outside tribe is strong enough to come in and conquer the decadent people.