They could save 1mm by going to 2.5mm jacks. Those are reasonably standard and would require only a small (and inexpensive!) adapter for older headphones. My Bose noise-canceling headset uses a 2.5mm plug/jack into the actual headphones (cable is removable).
I suspect the loss of this jack may be somewhat related to improving water resistance; those 3.5mm jacks are deep and have lots of potential for leaking.