“I had not remembered, until that time, how the Roman Empire rose, and brought peace and order, and lasted through so many centuries, until I forgot that things had ever been otherwise; and yet the Empire fell, and barbarians overran my city, and the learning that I had possessed was lost. The modern world became more fragile to my eyes; it was not the first modern world.”
I think the Romans, at least the more philosophical and intellectual ones, were perfectly well aware that this would happen to them eventually. After the fall of Carthage:
“Scipio, when he looked upon the city as it was utterly perishing and in the last throes of its complete destruction, is said to have shed tears and wept openly for his enemies. After being wrapped in thought for long, and realizing that all cities, nations, and authorities must, like men, meet their doom; that this happened to Ilium, once a prosperous city, to the empires of Assyria, Media, and Persia, the greatest of their time, and to Macedonia itself, the brilliance of which was so recent, either deliberately or the verses escaping him, he said:
A day will come when sacred Troy shall perish,
And Priam and his people shall be slain.
And when Polybius speaking with freedom to him, for he was his teacher, asked him what he meant by the words, they say that without any attempt at concealment he named his own country, for which he feared when he reflected on the fate of all things human. Polybius actually heard him and recalls it in his history.”—Appian, Punica
“I had not remembered, until that time, how the Roman Empire rose, and brought peace and order, and lasted through so many centuries, until I forgot that things had ever been otherwise; and yet the Empire fell, and barbarians overran my city, and the learning that I had possessed was lost. The modern world became more fragile to my eyes; it was not the first modern world.”
I think the Romans, at least the more philosophical and intellectual ones, were perfectly well aware that this would happen to them eventually. After the fall of Carthage:
“Scipio, when he looked upon the city as it was utterly perishing and in the last throes of its complete destruction, is said to have shed tears and wept openly for his enemies. After being wrapped in thought for long, and realizing that all cities, nations, and authorities must, like men, meet their doom; that this happened to Ilium, once a prosperous city, to the empires of Assyria, Media, and Persia, the greatest of their time, and to Macedonia itself, the brilliance of which was so recent, either deliberately or the verses escaping him, he said:
A day will come when sacred Troy shall perish,
And Priam and his people shall be slain.
And when Polybius speaking with freedom to him, for he was his teacher, asked him what he meant by the words, they say that without any attempt at concealment he named his own country, for which he feared when he reflected on the fate of all things human. Polybius actually heard him and recalls it in his history.”—Appian, Punica