Cutting the queue to an arbitrary number does nothing for wait times!
All it does is throw away information about the demand.
If there is a 15 hold limit, and it's fully occupied, we don't know whether the demand for the book is exactly 15, or whether it's actually 45, with an invisible backlog of 30 people having been turned away from being formally registered in the hold queue.
Those waiting to read the book now face an extra hurdle: they must repeatedly play a time-wasting lottery in order to obtain a spot in the queue. Someone who had been trying to get into the queue for weeks can have it sniped away by someone who gets in on the first try.
The only reason for trimming hold queues is if there is some resource problem; like they are running out of space in the database for tracking holds or something.