Shutting down service at night or even putting it at one train per hour will lead to a major economic breakdown. NYC is the city that never sleeps. If they were to just about or actually put the subway to sleep at night, it would ruin NYC nightlife for many.

I doubt it will happen as it would have to be an extreme last resort for them to shut it down completely. The money saved would evaporate from the cost of restarting it & shifting equipment & such. Plus I doubt they have the storage space to put all their cars away at once.