I‘d initially thought that eliminating fines was a great idea, but since then I’ve changed my opinions on it.
Perhaps a better way to go about it would be to have a month (or 3 months) long jubilee when people could return the books they haven’t returned in years without any penalties and then switch over to a more reasonable and less punitive fine structure. Having zero fines implies that there will always be people who exploit the system. Lower but non-zero fines will be closer to the optima, IMO.