When you build a new residential area, people move in with kids. After 15-20 years, kids move out, population collapses, and local services die. The best way to avoid this is building within / next to existing areas instead of building completely new areas.

Because then one is forced to go to what we dentist is there, whatever handful of restaurant are there, the grocery store that is there. No one wants zero or relative few choices like that, so trying to build mini self contained ecosystems isn’t going to work well

Nobody was forced but somehow something was missing, i tried to find reasons why these failed, it's a complex blend of mild poverty, mass social dynamics (same problems as in other neighborhood except it's not in higher concentrations and become unmanageable quick, broken window + reputation + tribalism). It devolved too fast to be handled. Even though on paper, they had best in class standards of living for average citizen (at that time my aunt lived in flat in Paris without toilets, it was the way for old building in her paris area; a year later she had electric kitchen appliance and her own toilet/bathroom).