While it’s hard to argue with your economics, it’s a lot more than nostalgia. I don’t think it’s a simple thing to sell a Shul to non Jews, especially as in the past many Shuls in Brooklyn ended up as churches l”a. (I don’t think this risk is as great with your average shtiebl, as there’s no real architectural value.)
And changing a neighborhood past a certain age may mean loss of friends, doctors, pharmacist, and of course familiar shul. Especially with married kids working most waking hours, it can literally be a health risk and perhaps even traumatic for some. Don’t belittle it.
No one talks of going back to Budapest, Lublin (or even Uman?) because we fled from there with the last inch of our lives, most of the infrastructure is gone anyway (not that it matters) and suicide is not a Jewish trait – anti-Semitism in much of Europe is transmitted with the mother’s milk. It doesn’t help that the American economy and standard of living is a lot higher anyway.
And to touch on the financial angle a bit, those moving to cheaper ESTABLISHED neighborhoods are for the most not funding the building of new schools. They’re for the most part financially struggling families, who in the course of achieving the American dream laid out their last penny and then some – even if the cost is a lot lower than in Brooklyn. Unless we have some private school tax, the communities being moved into have a hard time absorbing the cost of new schools, ASSUMING THEY WANT TO TO DO THE RIGHT THING.