America: What you are describing is very different than what occured in East Flatbush and Brownsville. You are basicly saying there is no room to grow in Flatbush, so the young couples are moving out. Nevertheless, Flatbush is not becoming less of a frum neighborhood. The current residents are staying put, and whenever a current resident does move, it is quickly filled by another frum family. The homes, when sold by a frum family, is being sold to another frum family. And Marine Park, which for all intents and purposes is an extension of frum Flatbush, is indeed growing by leaps and bounds with young couples. (Albeit a somewhat more modern crowd than Flatbush proper.)
Same with Boro Park and Williamsburg. There may, physically, be no room for growth. But the frum neighborhoods aren’t contracting (shrinking). And in both those neighborhoods, especially Williamsburg, there is in fact growth among the crowd of younger couples.