Why doesn't this apply to every minority? For example, when anti-Semitism broke down, why didn't it leave behind little Jewish ghettos of swirling social dysfunction and failure as the best Jews escaped into goyish society? Why not any Asian group? etc.

Good try anyway.

The origin of such ghettos can be traced back to segregation. Some of these communities thrived at a time and were fairly self-sufficient. The Black middle class fled these places. And all that was left behind were the poor and a crumbling society. The middle class Blacks might have served as role models to those less fortunate. The Whites didn’t care about them either. Everyone that could afford to get out, got out.

I think that hits the nail on the head. Evaporative cooling! When the segregation and anti-black sentiment are weakened but still in effect, of course it'd be the brightest, best socialized and most driven blacks who would break through the weakening celling, but the majority of blacks would remain an isolated community, suffering constant brain drain and hardly any "brain input" (from non-black assistance to the community, etc).

If this is model is correct then we should expect it to also work when dealing with class. If so it might explain the rise of the native British underclass, as the old culturally enforced class barriers where lessened by meritocracy in the early 20th century, evaporative cooling ensured the remaining lower class suffered more and more social pathologies.

If Charles Murray's Coming Apart case that such a cultural divergence between classes is taking place in America is correct, we should be able to make a few predictions about the near term social future of that country. At a glance these predictions seem plausible as they match most current recorded trends.

This seems related to matters discussed in my public draft on Meritocracy and the comment section there:

Let us leave aside problems with utilitarianism for the sake of argument and ask does this automatically mean we have a net gain in utility? The answer seems to be no. A transfer of wealth and quality of life not just from the less deserving to the more deserving but from the lower and lower middle class to the upper classes. If people basically get the position in society they deserve in life they are also costing people around them positive (or negative) externalities. Meritocratic societies have proven fabulously good at creating wealth and because of our impulses nearly all of them seem to have instututed expensive welfare programs. But consider what welfare is in the real world, a centralized attempt often lacking in feedback or flexibility, it can never match the local positive externalities of competent/nice/smart people solving problems they see around themselves. Those people simply don't exist any more in those social groups! If someone was trying to get pareto optimal solutions this seems incredibly silly and harmful!

With humans at least centralized efforts don't ever seem to be as efficient a way to help them as would just settling a good mix of talented poor with them.