The 19th century argument that the cost of living space would always rise as capital expands was definitively disproved in the 1960's and '70's when large swaths of Manhattan saw rental declines, in some places precipitous declines to zero and even below (landlord abandonment or arson, the city giving properties away to residents). This wasn't a shift of capital as with Detroit -- Detroit's experience gave support for Engels' warning that the immobility and economic inflexibility of home ownership for labor would be a curse as capital shifted locations, although in Engels' case he thought it wasn't so much capital shifting as that labor needed mobility to shift so it could sustain a strike by seeking work elsewhere.
In New York it was a cultural shift to the suburbs partly encouraged by government both by construction of suburbs and of infrastructure to take residents to and from the suburbs. That's why Robert Moses is so much blamed for the bankrupting of NYC. Capital did not shift to the suburbs, leading urban dwellers out of the city; capital was still in the urban center when Moses allowed the tax base to shift to the suburbs, and capital followed. It can't be blamed on the loss of manufacturing base: New York is growing in population and in wealth and tax base, but not in manufacturing. The move to the suburbs was a government-facilitated cultural shift that eventually spiraled the city downward as the eroded tax base undermined services, and middle class flight undermined public education. Explicit race-based programs like red lining and slum clearance closed the coffin.
The new demand for upscale housing shows distinct reflexes of its distinct functions. The quantity of apartment seekers will gentrify outer boroughs as long as central upscale development lags demand. The price of space will rise as long as the willingness and wealth is there. The draw in New York seems to be its density, safety and convenience. It's a party for the rich.
So why do all these rich folks come here and why are they willing to pay ever more? Is it the nightlife here? Or that NYC is the chain store capital of the US? Maybe it's just NYU.