There currently seems to be no shortage of startups in NYC, even without these tax breaks. And if a company only comes to NY because of the tax breaks, they might leave when the tax breaks expire.
I had the same thought until I realized that Human capital is a lot harder to move than buildings. Yes, it's possible to move a company away from a city, but 10 years is a long time. Long enough that a company would lose a ton of employees by moving out of state.