In the comments to my previous post, Citylights writes, "Lower Manhattan and Hudson Yards will have to accommodate the 1 million extra New Yorkers that are expected to live here by 2035. Because all other areas of the city are anti-growth."
Since I'm playing at urban planning with these posts I should say a bit about population projections. They're tricky things. I'm still not convinced that every town in the country is going to experience explosive population growth. Particularly not car-dependent places like Rockland County. Immigration is slowing, and birth rates are dropping.
But the walkable parts of the NYC region will probably see at least as much growth as is currently projected, from "sprawl refugees" fleeing the high cost of driving and the general unpleasantness of the strip-mall, cul-de-sac life. Still, I'm not really convinced that they all need to work in Midtown or Downtown, or live in the five boroughs. Instead, we should probably plan for the kind of multi-centered city that Jarrett described, because it's easier to grow. How big can you get when you still have to shoehorn most of the population onto a third of Manhattan Island every weekday?