I think a more realistic statement is that most of those buyers think they want the effects of economic segregation, and there is "real" subset of eg. multi-mil

I think a more realistic statement is that most of those buyers think they want the effects of economic segregation, and there is "real" subset of eg. multi-millionaires that actually want it. Most people don't want homelessness, "gangs"/crime , and rundown houses and other unsightly signs (perceived or real) of the economic lower-class in their neighborhood, a smaller subset don't want their children to socialize with the lower-class children, and a smaller subset truly want a private enclave of elitism. To be frank, there is a benefit to live near lower-income people: its easier to find service workers for your local cafe/restaurant/etc. The affordability problem facing many cities is that there are too few service workers because not enough people can afford to go to the city and work (because commuting from affordable areas isn't worth it). A city with a varied population can support more local businesses, and can have a more varied local economy, and can even be more resilient - eg. if there is a big storm, but the town has no plumbers, electricians, etc, you may need to pay more to "import" that workers from doing jobs in their own town (which also likely has residents in need of work). If you are a millionaire, you can pay to import service work, but most middle-class people can't.