[Sceptic]

Small scale communities are economically integrated, but need not be culturally integrated.

That's fine unless businesses strive to accomplish trade deals with nearby communities of different racial ethnicities, cultures or religions. If the communities is divided then there is a greater likelihood of discrimination against such trade deals to the point that communities would have to become more autosufficient.

[Eran]

I don't see why.

More and more trade is done internationally anyway. In fact, the benefits of trade are greatest when the trading parties are different from each other, each with its own relative advantages to offer the other.

Humans have consistently shown that, absent government intervention, they are remarkably capable of trading across all ethnic/national boundaries.