Da, i don’t think you’re familiar with Brooklyn demographics. Most fully modern people left to the 5 towns, Manhattan, and Teaneck. They used the eruv for the most part.
Talmidim of rabbi Shechter in Brooklyn(and I’ve met only a handful) aren’t sending their kids to the few remaining modern Brooklyn schools, i.e. yeshiva of Flatbush. They’re sending their kids to prospect park, kaminetz, etc.. Because they want an actual yeshiva, and not a coed Jewish public school.
As for chasidim, many, if not most at this point, carry. But mostly in boro park. The heimish Flatbush crowd is litvishe influenced and mostly does not carry.
But if you walk on shabbos in flatbush, you’ll see people carrying, and it’s identifiably modern people.