Residents moving out, unruly individuals moving in without taking up residence, more residents move out, then more criminals move in. The cycle repeats itself nonstop and unabated, until finally, there is virtually nothing left but vacant dilapidated properties with squatters engaged in regular criminality.
The situation sometimes spreads like a disease, increasing the size of the area deteriorated.
It is time, and been time, for that scenario to stop, and those neighborhoods that have gone through that, to repair.