I think this just matches human behavior a lot more too in terms of how late books happen - usually just because someone forgot or it was inconvenient to return

I think this just matches human behavior a lot more too in terms of how late books happen - usually just because someone forgot or it was inconvenient to return it on time. Eliminating late fines and instead blocking checking out more books means that the person is incentivized to bring the book with them next time they go to the library, so that they are rewarded with being allowed check something else out. Late fines tend to do the opposite... once they realize the book is past due, they are incentivized to avoid going back to the library, because once they do they'll be punished. The incentive of being able to check out another book is likely not enough to overcome having to pay even a relatively small fine. It seems fairly intuitive to me that this creates a perverse incentive that will often discourage people from returning books at all, rather than encouraging returning them on time. As mentioned elsewhere, this seems like the same discovery Netflix made back when they were mailing DVDs: it is more effective to incentivize returning by making it a tit-for-tat trade than by fining non-returners. Sure, this approach might end up with people never returning items if they never want to get more (move away, etc), but that's a relatively uncommon situation for a library, and doesn't seem to have become a major problem for Netflix. More subjectively, I also think that charging a replacement fee to patrons that truly cannot return the book feels more fair than charging a per-day late fine until you hit replacement. It's more of a direct indemnity sort of situation.