These comments and similar do surprise me. I fly from time to time UK to Australia, and I can't remember the last time I even turned the IFE on. Headsets always get handed back still in their plastic bags. Looking around during the collection, I am by no means unique either.
I would agree with you to some point. With apple creating ever sort of media in your hand, less people will be using it.
However, if you look at the investment that airlines make in their hardware there is clearly a large percentage of passengers that use the services.
Maybe people giving them back in plastic bags is something to do with them using their own earphones (going back to my first point) or have earphones from previous flights?