Sidebars are my favourite!

Going by the model presented as the example, there would eventually be a large homogenous group which has fragments of the best elements of all the group which it superseded. Certain aspects of the many prior cultures which make up that centred society around that group would be there to learn from, adapt from and strive from.

The biggest threats to that group (Outside of environmental changes) would be other groups of that size (Or bigger) or a tiny element which is completely new and unaccounted for in the entire heritage of what had made up the group.

This tiny element is sometimes thought of as a virus which causes the group's demise. In cultural anthropology the concept of a meme pathogen bringing down a civilisation is far less likely than another group doing so. But in theory it is possible.

Either the elites get it right and society will continue without evolving or ideas will get into the system and alter what the elite group does to continue controlling it.

This is all just an example for purpose though. I'm sure there are many factors missing which we can't account for yet. I'm just suggesting that the 'divide and conquer' could either be an external factor or an internal one as it has always worked as a form of attack for the largest groups in the West.