Comment by t_mann

Exactly. Even considered on its own it's a really bad example of rapid collapse. Roman civilization lasted for about 1000 years until the collapse of the Western Roman Empire in 476 AD (250 more if you include the pre-republican period). One could argue at length when its decline started, but a reasonable starting point would be ~250 years prior to that, with the great crisis of the third century. That's a quarter (fifth) of its entire existence, and a lot of human generations, who would mainly have noticed significant up- or down-swings during their own lifetimes (with somewhat equal probability). Only the most historically literate would maybe have perceived a general downward tendency over the centuries.