Just my opinion but I think Simple Delivery is equally to blame for the drop off.  There are still plenty of people who, although they have smart phones, may not want to experiment with QL codes or lockers, taking pot-luck as to who delivers purchases and seemingly hit and miss arrangements as to who is responsible for refunds when purchases arrive damaged or not at all.

Yes, all these things are (mostly) avoidable for the knowledgeable who use ebay regularly, read the boards or have extensive knowledge of the Help pages.  But again ebay has taken something that was easy and familiar, like reading how the seller said they would arrange for delivery and turned it on its head causing uncertainty and confusion.

Another of those old retail 'sayings'  --  Markets hate uncertainty.