Similar but slightly different: what stops you from borrowing a few books and then keeping them, not for forever, but for an unreasonably long time?
Physical books are a limited resource, and others might like access to them. (It's the same reasoning as for parking meters and 2-hour parking: make sure someone else gets a turn.)
Without any time limit on books, the only thing motivating you to return a book is that you need some other book, which might happen weeks later or maybe 6 months later.
The library will continue to charge patrons replacement costs after 30 days overdue. Not sure about NYPL, but in many libraries, this will be waived when you actually return the item. You would just pay the fine (capped at $5 most places I've worked). Presumably the letters and notices of overdue books offer the same motivation to return books now as they did before, the only difference is you won't be charged anything upon return.
Similar but slightly different: what stops you from borrowing a few books and then keeping them, not for forever, but for an unreasonably long time?
Physical books are a limited resource, and others might like access to them. (It's the same reasoning as for parking meters and 2-hour parking: make sure someone else gets a turn.)
Without any time limit on books, the only thing motivating you to return a book is that you need some other book, which might happen weeks later or maybe 6 months later.