I’m not qualified to comment on the evolution of this term from a historiographical perspective. However, as someone who has analyzed “decadent” in the common tongue out of amusement, I can’t resist commenting.

One reason for my curiosity in this term is due to its amorphous connotation: its literal, baseline, definition (self-indulgence) can manifest positively and/or negatively depending on the context. To make things more complicated, It could be used with one connotation by the speaker of the term and perceived in the other by the listener. A “decadent chocolate cake” would generally be viewed favorably, whereas a “decadent leader” looked at with suspicion.

To your point, this is where the use of the term hinges entirely on the perspective and worldview of the individual using the term with sufficient context. The term is inherently a value judgment. An historian using the term in a derogatory way (negative connotation) to highlight the selfishness of an age is by default inserting their worldview into their analysis of that time. The term assigns a value based on that individuals spectrum of acceptable norms. It’s unavoidable. My conclusion is the term’s value as a tool of dispassionate analysis is limited. It merely functions as a marker of an impending worldview, or as a throwaway “fluff” term that acts as an appendage to “cake”, “vacation”, etc.