> Chang established the basis of this research in a previous publication with an intentionally simplified model that ignored such complexities as geography and migration
This seems like a very impactful simplification. Especially given the relative limited mobility of peoples up until very recently.
What is your evidence to support that? there is no 1:1 comparison here. The groups that likely survived these collapses were largely nomadic or at the fringes of society, used to sustaining themselves without the support of the existing power structures.