Eh, I grew up with the faces on TV constantly blabbering about the "decadent West" and yet my country and most of the ex-USSR has deteriorated worse than any western one. My eyes kind of roll every time someone says decadent society as it reminds me of communist propaganda. Not seeing much of those four horsemen, either.
Decadence as a concept makes a good polemic but it isn't a good source of judging problems let alone solving them. It really just means "I the self-proclaimed moral authority dislike this direction of society!".
The closest to an "objective decadence-like concept" is an expectation of a way of life that cannot be sustained without significant damage. Cottage industry after the dawn of industrializarion would be "decadent" as factory volume beats them out and can no longer sustain the middle class lifestyle. Not exactly what comes to mix when one thinks decadence but it is a lifestyle made unsustainable. There is no "sin" to it but it just doesn't work anymore and trying to force it wouldn't work out well. Notably even that is subjective and depends upon means. Having less than 10% of the population involved in food production would be stupidly unsustainable before agricultural advances but is the first world norm today.