Yeah, see, that's the thing: you're making a pretty big fallacy assuming that the costs of providing overnight service actually impact maintenance work in any meaningful way.
As it turns out, overnight service is relatively inexpensive to provide, and temporary disruptions to evening or weekend service for maintenance (which already happen) are sufficient to address most maintenance problems that can't be handled during working hours. Cutting overnight service wouldn't solve the problems that are actually preventing the maintenance that's needed.
I'm not going to bother to go into more specifics, because they're well documented, not just in this comment thread but also the exact original article. If you're interested, those are both excellent places to start.
It's your responsibility to show that the saved maintenance cost is hurting the "essentials" of NYC subway.