“Decadent” not in the sense of “overly indulging in hedonistic sensual pleasures,” but in the sense of (quoting from the intro): “economic stagnation, institutional decay, and cultural and intellectual exhaustion at a high level of material prosperity and technological development.” Douthat says that the US has been in a period of decadence since about 1970, which seems about right and matches with observations of technological stagnation. He quotes Jacques Barzun (From Dawn to Decadence) as saying that a decadent society is “peculiarly restless, for it sees no clear lines of advance,” which I think describes the US today.