If the power was going to be out for months, and no chance of access to water or a working sewage system, then yes you'd want to evacuate NYC, perhaps not all o

If the power was going to be out for months, and no chance of access to water or a working sewage system, then yes you'd want to evacuate NYC, perhaps not all of those 8 million people, but anyone without alternative means of disposing of their human waste. As for how to get water to people, you could use trucks to deliver water tanks. You could also have small scale electricity production through solar and wind and mobile electricity generators. It'd require considerable effort to pull off, but we would have options.