I've heard elsewhere that it's not really New Yorkers buying these condos. They described them on Morning Edition as "the world's most expensive safe deposit boxes." Apparently, if you're super rich in the Mid East, Russia, or Asia, it's a lot easier to buy a $10M condo in NY, and use it to park your art and jewelry than to have it unprotected in your home country. They don't free up space for anyone when they're neither moving population around, nor adding population to, the city. In fact, the Times had a story about the penthouse in 432, It's a $95M behemoth that was bought by a couple hedge fund managers as a place to have parties occasionally.
Heard the same on NPR. And they keep building these, but there isn't really any demand for them. So now there are a bunch of really expensive apartments that nobody's actually living in, next too a bunch of really expensive apartments that nobody's using.