Well, I'm kinda torn between "worn and tired" and "well, actually not-so-hard to believe" depending on one's position on the Fermi Paradoxon and the basic chances of older civilizations, given the age of our galaxy, and what your expectation of a civilization's life span are. In a way it ties in with the "best bet" we can make on the one hand, and deep-rooted psychological and cultural predispositions; psychological in that most people adopt an "everything was better back in the days" attitude as they grow older, and culturally in that for example the middle ages still had a faint memory of the glory of the Roman Empire when everything had crashed down, suggesting that civilization was on a downward trajectory from some mythical golden age of the ancients (or stagnant at best). Which are, IMO, contributing factors why this trope works so well (and will never die).