Indeed. They don't even argue that the factors that led to decline in the observed locations are peculiar to those locations (and therefore won't lead to later

Indeed. They don't even argue that the factors that led to decline in the observed locations are peculiar to those locations (and therefore won't lead to later/slower decline elsewhere). Rereading, they don't even advance any theories that such geographically limited factors might exist, they just take them for granted.