As Deaths Outpace Births, Cities Adjust - New York Times:

While natural decrease occurred in many Southern cities that were magnets for retirees, the overall population was replenished by the influx of younger migrants. But in Pittsburgh and other places outside the South, not only has the population aged in place, but also, to a lesser extent, the very old — often disabled and widowed — have returned to spend their last years with children and grandchildren and avail themselves of better medical treatment and transportation.

Hmmm....I don't think Pittsburgh will shrink to nothing though.