That's interesting, how this guy focuses on the city I work in, and the city right up the road from me which I have visited numerous times over the years. When I first moved to DC in 1985, within the first year of my arrival there was indeed a murder one night, right next to my building.
Interestingly I lived for a few months prior to that across the Anacostia River, which if you know this area makes you go "Huh?" Anacostia is Washingtonian for "Harlem".
And while there are probably plenty of parts of Harlem that doen't comport with the televised and word-of-mouth reputation of Harlem, and while there was a literary movement of great import born in Harlem, you still expect a harder time in Harlem than you do in some trendier area (Upper West Side? Do I have that right?)
Well, I moved from Anacostia, which was supposed to be the Heart of D.C. Darkness, to a trendier area in NorthWest. (We also delineate liveability by quadrant: Southeast is no good. Northwest is very good.) Like I say, within a couple months, they found a murder victim in the alley next to the building.
Now, that's just a somewhat disturbing part of life in the cities. Violent crime waxes and wanes. It usually shoots up along with unemployment and poverty, so for the time being, if you're of a mind to be paranoid, I'd be moreso.
As to the barbarians-at-the-gates underlying premise (or perhaps more accurately barbarians-within-the-gates,) fail. I've lived in cities or close-in suburbs my entire post-college life, and the fantasies spun by those in the exurbs capture only the phenomenon of violent crimes which have always been part of urban life. You never stop having to fight the fight to keep the risk down. You also never stopped having the need for cities, up until now (although I will say that the advent of computer remote networking makes the effective span of a city worldwide, so it is quite possible that cities are on the way out.)
Once the affluent no longer need population concentrations to peddle goods in brick-and-mortar ways, one can easily envision a ramped-up decentralization, where the hub is less important than the suburbs around it, then becomes only recognizable as the formerly-important middle city out of several in a relatively tight-knit region. The exception will be the industrial city, which of course is already a greatly eviscerated phenomenon. None of them will ever again grow to be a region-dominating behemoth.
I love watching change unfold; in a way, we're all blessed by this today. We can watch changes that once took hundreds of years happen in decades. Of course, the same applies to things like, oh I dunno, the earth's warming, the collapse of civilization, whatever.
But then, life gets dull without an element of risk, right?