Frederick Law Olmsted realized early on that even a huge project like Central Park could be funded from the increase in land value it created in the neighborhoods surrounding it.
Central Park worked because the existing density was already high and there was no way for Manhattan to grow outward (dredging excepted).
Central Park was built before the first subway and before the bridges connecting Brooklyn, Queens and Jersey. The dirt dug from the tunnels was added to lower Manhattan. Prior to elevators buildings rarely went higher than 5 stories. There was in fact ways for Manhattan to grow outwards (effectively).
Astor and others benefited as population expanded north.