Then the productive members of society move out.
Why is that?
I pass by some drug dealers sometimes on the way to work. I don't see the problem. Occasionally I get asked if I want to buy some drugs. I don't want to buy drugs so nothing else happens.
Drug dealers are criminals. Criminals commit other crimes and attract other criminals and crime. For example, rival drug dealers who want to take their spot and use violence.
The way to solve these issues is legalization and regulation.
I have to admit, I'm not really following the logic there.
Drug crimes and non-drug crimes (assault, robbery, x/y/z) commonly cluster[1], and citizens with working risk estimation skills move on and cede the space.