Amazon would have to first hire those people. And the NYC labor market is already pretty efficient and saturated. It would basically push a lot of IT and related job wages up, for not much direct benefit - other than dumping a big cashflow on the city. (Which pushes up real estate and other prices.)
I'm not saying having HQ2 there would have no positive effects, it'd have absolutely contributed to the growth of the tech scene. But it'd have probably accelerated the arrival of other problems, which is something the city might not be ready to tackle.
