There are interesting ideas here, but also a lot of presumption. Just because a people lacked stone writing and centralisation doesn't mean they were free of local tyrants and internecine tribal warfare.
I was thinking the same thing. I don't think there's evidence of anyone ever trying to flee from an Empire where they lived, at least if they were not there as slaves brought from another area. If the "outside" was so great, you would expect people to try to move there... everything I've read so far makes me believe the exact opposite has been true. People tend to want to move to more prosperous areas, just check the world today.