Toward Ecosophy

This is a quite sobering assessment, but I don't think it's wrong, and it's on the level of what happened to the Western Roman Empire in its later centuries, when the actually Roman population likely declined by as much as you said here. In particular, by about the middle of the 5th century, there were hardly enough Romans left to defend the Western part of the empire, so it was probably on the order of a 90% drop by that time. It was Germanic barbarians ruling over landless peasants working the giant estates of the remaining aristocrats at that point - of course, this was the basis of the dark age to come. I don't see something like your scenario as at all far-fetched these days. Honestly, the condition of the big cities in America has the feeling of a civilization in twilight, not unlike how Europe probably felt around the time of Peladan (with the notable exception that Europe had a large youth population in those days and we do not).