Once a neighborhood reaches a “slum equilibrium” it becomes very difficult to pull out of it. People don’t walk around so there are no “eyes on the street.” Stores find it hard to stay in business, so there are few jobs. The most together families tend to move away. A general atmosphere of disarray may, itself, contribute to increasing levels of criminality (per “broken windows” theory), and as the neighborhood becomes poorer and less desirable it loses political clout which makes all the problems worse. The good news, such as it is, for cities is that there’s reason to believe that the coming decline will see more in the way of the slumification of the exurbs than it will re-slumification of “transitional” neighborhoods in comeback cities. For example, when I was growing up there were unsafe areas in the East Village that I think have now clearly passed into a safe “non-slum” equilibrium with plenty of businesses open and eyes on the street, where if real estate prices fall there are plenty of non-poor people who’d be eager to take up the slack.
Things were getting worrisome over the summer when we had people pulling copper pipes out of five vacants around our house, and some of the neighborhood kids had decided they'd make good target practice for their new paintball guns. Since then, most of the houses have been bought and are in some stage of re-repair and habitation, so things haven't fallen apart yet. There are actually more kids playing football and skateboarding in the street than there were when we moved here.
In particular, what seems to have happened so far is not "the most together families" moving away, but the least together families losing their homes and the most together local immigrant families buying low.
We don't seem to have been hit yet by a new wave of absentee speculators/slumlords. It was just within the past couple years that a particular bad landlord who had bought up a bunch of houses on Adelaide presumably in the 70's finally gave them up, thus allowing a key chunk of the neighborhood to be improved. OTOH, some kind of real estate junket of white people was walking around the neighborhood yesterday. I don't know if that's good news or bad.