Where did the people go who created the vast areas of urban abandonment that have been there for years?  That's fairly easy to explain.  They went to the suburbs, or moved to other towns that were booming.  Cities in the Midwest generally peaked between 1950 and 1970, while the South, the Southwest, and the West began their ascent later on.

It's not uniform.  Detroit proper peaked at the 1950 Census at roughly 1.8m, and has fallen by about half since then.  However, the metro area continued to grow, and the Detroit metro area is today larger than it was in 1950.

Even in boom towns, a similar process took place.  Atlanta hit a peak in 1970 at a little south of 500k within the city limits, and dropped about 20% of it (to ~394k) by 1990.  In the last 20 years, though, the city proper has recovered and grown to 520k.  Meanwhile, over that 40-year period, the metro area grew from about 1.7m to about 5.6m.

Flint, on the other hand, simply collapsed.  It isn't big enough to be a Detroit, let alone a New York or DC, where eventually demographics, cultural preferences and economic power begin to shift people back to the cities, as they have in those two cases.  And with most of the population leaving Flint, there's no obvious road to recovery there.

Many cities are seeing that shift, though.  Even ones which are in rough shape, and here I'm, of course, thinking of good ol' Baltiless.  I knock Baltiless a lot, but it's now in the early stages of gentrifying.  But Baltiless has the advantage of being surrounded by wealthier areas, which guarantees some money will flow into it.  It's got the makings of a decent economic foundation (Johns Hopkins, U of Maryland, some good tourist traps, etc).  And it has the advantage of being dirt cheap compared with everything nearby.

Now, what becomes of the new abandoned people and places?  That's another question.

Be nice to America. Or we'll bring democracy to your country.