It seems fairly obvious that if you condense the super-rich into smaller and smaller areas (trick them into spending more money on less space, as these skycondos are essentially doing), then logically there is more housing space left for everyone else. More space, less competition, lower prices. That's theoretically the domino effect. So the super-rich flee to the sky, the medium-super-rich have more options on Long Island, the pretty-fucking-rich move back into the rest of Manhattan and suddenly Brooklyn is a bit cheaper. Or something. On paper it makes sense to me. In practice, you've identified a problem which I agree with -- it's too abrupt. Also, this sort of thing assumes a trivial cost of moving, which is false. But over a long time, it may have an effect on the composition of neighborhoods.