The point is that this is a city with a large majority of blacks, and it just so happens to have terrible police. That isn't a contingent circumstance.
Everyone with money left well before 1990, the vast majority of white flight took place in the 60s, 70s, and 80s
Edit sp332, lol yes 2/3 of the white population remaining in 1990 left after that, the problem with your line of thinking is 86% of the white population had already fled by 1990. 2/3 of 14% is not much...