What absence of smaller disasters? Why don't the brushes with nuclear war and other things you mention count?

Also, civilizations have fallen. Not in the sense of their genes dying out [1] but in the sense of losing the technological level they previously had. The end of the Islamic Golden Age, China after the 15th century, the fall of Rome (I remember reading one statistic that said that the glass production peak during the Roman empire wasn't surpassed until the 19th century, and it wasn't from lack of demand.)

[1] unless you count the Neanderthals, which were probably more intelligent than h. sapiens. And all the other species in genus homo.