An ethnically homogenous area doesn't always imply it's a ghetto though?
This doesn't mean that ethic areas just melt away, though. Chinatown can remain Chinatown on the basis that they value being close to Chinese community centres and Chinese grocery stores more than the average resident, meaning they're willing to pay more and price out other people to live there. They can't remain Chinatown by literally refusing to rent to someone because they're black, though.
This allows ethic areas to remain if they provide a benefit to their community, but melt away to redistribute real-estate resources more efficiently if they don't.
When there's tons of cheap housing available, then having small areas with expensive pricing isn't per se a problem (but could be an issue in context, say a small midwestern town excluding black people from buying houses by over-inflating prices.)
You can, for the most part, if you don't make it harder than it needs to be to build apartment complexes.
You can't replace one house with 16 apartments and expect it to work as well.
Parking, noise, litter, etc. It's not fair for a developer to push their direct costs onto the neighborhood instead of eating it themselves.