[edale]

It's an interesting question. I imagine it would look a lot like Chicago does now. I would assume that most of Manhattan would be pretty gentrified, at least south of Harlem, but places like the Bronx and perhaps portions of Brooklyn would look more like the South Side of Chicago. The city would have probably more or less 'consolidated' into Manhattan, and you'd have the classic tale of two cities between it and the outer boroughs.

If the city never really recovered from its low point in the 70s, I'd imagine more corporations would have fled the city, either to the burbs or to other regions of the country, and we could have experienced something similar to the shift in power between Montreal and Toronto. But that's hard to even imagine, seeing how dominant NYC is in banking, finance, etc.