I think that a majority of the future generation frum people will not be staying in Brooklyn, they’ll be moving mostly to NJ/Monsey/FR & Five Towns.
The only people who’ll stay are the chassidish crowd who will take over Flatbush (and make it a 2nd BP).
There are some in the Litvish/Modern crowd who are trying to move into Marine Park and even Mill Basin for the cheaper housing. However, as i mentioned before the future generation will be leaving Brooklyn.