It’s a historical trope that societies that are “decadent” (in moral decline) are also in temporal decline. It’s a story that’s told of rich, liberal societies with indulgent mores. More austere (poorer, less liberal) societies are conversely honored for their purity; they are more likely to be romanticized as heroic ages.
BUT richer, more liberal societies are also more likely to be literate, to approve or allow the writings of a broader range of opinions, to support writers of satire and other kinds of internal criticism. So these societies will leave a profusion of critical writings, seemingly documenting their relative decadence. But less-lettered, more censorious societies may have all the same moral failings and simply fail to document them. To the extent that the seeming relative turpitude of rich, liberal societies is the product of the richness of the record they leave, the conventional wisdom about “decadent” societies is undermined.