I don't see how ejecting anyone follows from the premise? The idea is just vaguely that wealth imbalances between arbitrary groups could be beneficial overall.

I don't see how ejecting anyone follows from the premise? The idea is just vaguely that wealth imbalances between arbitrary groups could be beneficial overall. That in itself doesn't specify anything about the size of those groups, or who should be in them. And if we were setting out to create such arbitrary divisions, geography seems like a fairly practical way to do it. It's easy to keep people in separate areas separate, whereas segregating currently integrated races would be a significant undertaking.