Sub-70IQ retards don't really cost society too much. They don't commit crimes like the 80-90IQ cohort does and they're typically the victims of violence, not the instigators. Sure, they're a drain on welfare, but it's typically people significantly smarter than them that clog up the prison system and commit the welfare fraud.
You have to separate the congenital retards (have a physical disorder that causes their retardation) out from the non-congential retards too. Down Syndrome kids just get a dolloping of NEETbux, often doing small tasks like helping at the fish and chip shop. For white and asian populations, most congenital retards die off pretty young too, like around 40, and almost never sire children.
Non-congenital retards cause a bit more crime, are a bit more violent, sometimes have more kids, but not really as much as people a bit smarter than them in all of those ways. Blacks have more non-congenital retards relative to congenital retards. And here again, they're mostly victimized in their community.
Anti-social criminals typically aren't the stupidest people in a given society.