This isn't really a problem for whites in the US because whites just pay a premium to live in the "good school district" (which is really just a code word for "white neighborhood"). Maybe things are different in South Africa. I think if White South Africans had aimed for a more natural, free market type of segregation like exists in the US, instead of a legal one like they implemented in Apartheid, then South Africa wouldn't have become unlivable for whites as soon as the restrictions on blacks were removed.
Back to the original point, maybe white South Africans dislike living among black South Africans enough that it is enough to get white South Africans to appreciate each other. The cohesiveness of the Confederados did not last, but maybe the racial differences between white and black South Africans is greater than the difference between Confederados and the surrounding mestizo populations.
As for the OP about separating from mainstream and forming an insular community, I don't know how many people are disgusted on a visceral, sensory level like the white south africans are of black south africans. I mean, I would not like living in a big city these days because if you go into a coffeeshop in an upscale part of Manhattan you will find the staff to be superficially friendly but unbearably passive aggressive behind the smile. And in a ghetto one might find the people obnoxious or dangerous. But if someone wants to get away from either the passive aggressive douchebags or the ghetto thugs they can just move to upper class suburban neighborhood in middle America, where they are not as common. One may have philosophical disagreements with these people, but this is not enough to get people to segregate, because segregation must be motivated by visceral disgust at the sensory level (e.g. you don't like how they look, sound, or smell, or perhaps don't like that they tyrannize you).