To say the decrease of young people in any community is not a problem is highly inaccurate. Many aging parents will eventually find it hard if their kids are a few hours drive away. And many of them will not want or won’t be able to move away from the neighborhood they called home for decades.
Add to this the huge Jewish infrastructure that may need to be sold to non-Jews if this trend exacerbates. And then there’s the huge cost (not just financial) of literally replacing those Shuls, Mikvaos & schools elsewhere, which btw is partly to blame for the “school crisis” in rapidly expanding communities like Lakewood and Monsey.
Plus tuition for those that remain might need to go up if the student population shrinks. If schools must close, parents will have fewer choices and children may be effected negatively if they are forced to switch schools, away from the schools they were doing well in.
If what the OP is saying about Flatbush is true, it may be better off than other Brooklyn neighborhoods where Gentiles and secular Jews may take over, because some of the infrastructure can hopefully be saved and put to good use.