> These societies hit scaling limits precisely because they failed to diversify.

Neither of them hit 'scaling limits'. The Phoenicians made a strategic mistake at the outset of the Punic wars, and Viet Nam took the Mekong delta just 150 years ago, almost entirely wiping out the Khmer Krom. Shortly afterwards, international politics had changed so much that aggressive territorial expansion stopped being profitable.

> One can deprioritize the points of difference

Which is assimilation. Think about what deprioritization actually entails. Prioritization of language for example. Deprioritization means that you cannot have a person who speaks Sicilian fluently, and English at an A2 level. As a natural consequence, Sicilian as a language dies in that family after 3-4 generations have passed as a natural effect of no longer being the higher priority language. This happens across all cultural axes. Speaking as someone whose descended almost entirely from early Syrian immigrants to America. Deprioritizing identity means giving up that identity to every degree that matters.

> Romans Italians and later provincials into their citizenry.

Roman power was established at a time when their diversity meant gangpressing non-Latins into conscription and not giving them citizenry afterwards. While they eventually began extending citizenry to conquered people, and later even equated Romanness with citizenry, much of their diverse populations like the Germanics, the Celts, the Levantines, the Maghrebis, never cared about Rome or Romanness. The Germanics even destroyed Rome, and none of the rest cared when it all went up in flames. It was just an annoying exploiter, one in a long line of many. I think the scaling limit point is interesting because Rome explicitly did hit a scaling limit at the peak of their diversity and shortly afterwards collapsed into hell on earth. Cherry on top of Anatolians and Germanics running away with their identity because it had become completely meaningless.