I'm not yet finished the book, but it is explaining the down cycle by going back in history. According to the author, we were not a hunter/gatherer culture 10,000 years ago.

Rather we had advanced mental abilities, so we didn't need that much technology. The author cites recent scientific discovies, like Göbekli Tepe, to make his point. We had advanced abilities--and we slowly lost them.

As for the future downcycle, I'm not that far in the book.