> Your comment assumes a very negative interpretation of what I said.
Not sure that it does :D
I'll address your points one by one:
> Boundaries are an expression of each tribe exerting an area of influence.
Modern boundaries, like modern nation-states, don't really work like tribes did. Prehistoric tribes tended to move around, didn't build fences or patrol them with armed guards and usually had a relatively vague concept of ownership.
In many places around the world boundaries were drawn by colonial powers according to their own interests and not by natives themselves. For that reason borders very often actually split cultures and lead to ethnic tensions (e.g. in the Middle East).
> we don’t want to eliminate human ethnic/cultural diversity ...
Fully agree.
> ... and that requires boundaries which enforce rate limited exchange
I really don't understand why you think this is the case. It seems to me you're operating with a mechanistic/thermodynamic model about how cultures work. It's simply not true that you need borders to allow diversity to survive.
Take for example the concept of linguistic continuum: https://en.wikipedia.org/wiki/Dialect_continuum. This actually ante-dates modern borders and shows how you can have cultural diversity and cultural mixture at the same time.
> Intermingling at a natural rate which allows each tribe to maintain its integrity is natural, and has evidence going back longer than writing.
Again, tribes and cultures used to mix all the time and modern states are not like tribes (or ancient kingdoms or empires for that matter).
> Brexit was an act of political violence because the UK immigration exceeded the rate at which the society could naturalize immigrants.
While I agree that Brexit was caused by a perceived flood of immigrants, the act of violence is mostly self-harm. The public's belief that immigration is excessive and harmful was encouraged by certain political and economic interests and is not an objective fact.
> Homogenizing everyone into a single global culture, which would be required to have a single government without boundaries that rate limit migration
I wasn't arguing for a global government to begin with. However, again, I don't see why you think administrative unity requires cultural uniformity. We can, and do, have institutions that oversee multiple diverse cultures, both across nation-states (e.g. UN, EU, NATO etc.) as well as inside them (many countries are federations that oversee ethnically diverse populations).