Realistically speaking Flatbush like all frum neighborhoods (including Lakewood one day) will wither. That is just the way the world works. America always has changing demographics in all frum neighborhoods. In the 1970s when Boro Park and Flatbush was the new “in” neighborhoods and shuls and school building in the old neighborhoods were closing down and being abandoned people thought it would stay that way forever too.
In a place like Flatbush where there is no predominant Askenazi group that is determined to stay and keep their Kehila there, the factors causing people to move ultimately will have to prevail.
The question is how long will the process take.
Thirty years ago during the Dinkins days, I and everyone I knew, was sure there would be no significant frum community in Brooklyn today. We were also sure that neighborhoods like The Lower East Side or Washington Heights would be completely gone. We were clearly wrong.
Therefore I predict that Brooklyn will go the same way The Lower East Side did. Losing frum/Jewish population for almost a hundred years but still having people who are interested in staying.
Side note. In the 1970s when I went to the LES everyone seemed old back then. Presumably, those people are no longer alive. Yet the LES is still around.