The problem with the way the question is answered is that it is quite broad but after some limitations I can try to answer it. Some clarifications are necessary before diving in the specific answers:

The Roman society was very traditionalist in it's essence. The republic had no constitution and all worked because the politicians could base their actions and what was done before. As a sidenote it's also one of the reasons why the political decisions of the end of the republic like Pompey ruling Spain during his proconsulate without leaving Rome, sending legates instead. It created a precedent for the imperial system (in which the emperor as proconsular power on all the imperial provinces but send legates instead). All of that system that was entirely dependent on roman political history. All of those previous actions constituted the Mos Maiorum: the customs of the ancients.

So in the late republic and in the early imperial period we have some authors that are nostalgic of the past... but only on a moral standpoint, because they couldn't deny that Rome ruled the world. That idea is derived from a biological vision of history in which a state has 3 or 4 phases: Infancy - youth - maturity - old age As you may have guessed the writer always writes in an era of old age so he has every reason to find the times amorals but to avoid criticizing the emperor the idea of a new youth was even introduced.

Now for late imperial romans the melancholy is tempered by the coming of christianity, it created a chasm in history that enabled to see the present as better bacause it was blessed by christian revelation. Now for late roman pagans the situtation was entirely different, their temples were progressively closed and the number of people willing to make sacrifices plumetted and they became outlawed by the late 4th century. So the accusation that christianity caused the fall of Rome in 410, is understandable and Augustine had to write the city of God to answer those criticisms.