Many narratives of civilizational decline focus on some kind of moral decay, or decadence, as a primary causal factor. But "decadence" is often only very loosely defined, relying on cherry-picked data and the biases of the author - Glubb's "The Fate of Empires" being a preeminent example, along with essentially the entire genre of Roman Decadence analysis.
Have there been any rigorous attempts to formally define "decadence," that explain what it is, where it comes from, and why it causes empires to collapse?
Since nobody has (yet) taken a stab at addressing your question I'd like to provide some feedback that can hopefully get the ball rolling for you. This will be in bullet form as the point is not to offer a narrative, but rather some technical critique of the question itself. To this end, please don't interpret any terseness in my language as contempt -- and please interpret everything as literally as possible (i.e. without subtext). To wit,