What you mustn't forget is the role Roman writers themselves played in creating that story. Criticising the moral failings of the present and holding up an idealised past as a foil was an established genre of literature, and almost everyone whose writings survive did it. It probably wasn't that the Romans were particularly decadent, but they themselves *thought* they were (much as modern Western society for some reason is convinced it's particularly shallow, heartless, violent and crime-ridden). Martialis, Juvenal, Apuleius and Petronius painted a broad canvas of mad diffusion and pointless monery-grubbing, Livy and Tacitus firmly established the idea that the ancestors were infinitely superior to the present, and of course the major Christian writers that shaped Medieval and Renaissance reception such as Orosius happily picked up that narrative. Roman literature was deeply convinced that the moral bearings of its society were lost, all the while suppoerting one of the most fascinating and influential discourses on morality the world has ever seen.