I'm not sure what sort of decline is happening here. Silicon alley is booming. The city is a cultural smogasboard. Hundreds (maybe thousands) of large corporations are centered and based here. Construction is rampant. And the city is likely one of the safest metropolis's in the world.
But maybe you could prove me wrong... I'm not sure, just noting what I see.
The economy of NYC is much less diversified than it was say 20 years ago or in the postwar period.
Those corporate HQs used to come with thousands of workers -- now more likely a couple floors of folks supporting the CEO. A lot of residential construction is funded by capital flight and shady political lobbying that helped get 2/3 top state leaders convicted of corruption charges.
Still a great city, but not the economic behemoth that it was.
But maybe you could prove me wrong... I'm not sure, just noting what I see.