[yummyfajitas]

This is an argument I find semi-convincing, and not solely because the economics is wrong ("deskilling" isn't an externality and education is not a public good).

Suppose that integration of black Americans also allows them to produce negative externalities and consume positive ones produced by white Americans. For example, suppose black Americans are disproportionate consumers of welfare and producers of crime.

If externalities are your true justification (rather than a post-hoc rationalization), then we should segregate black Americans as well. Do you favor this, contingent on circumstances?

Your example of Germany is interesting, given that the GDR did in fact prevent emigration in order to make sure the state didn't lose it's investment in human capital. So if the US opens it's borders, the Germans can just rebuild their wall.