You're implying it's because of cost-savings, but OEMs still typically offer 3.5 mm jacks on their lower-end models (even waterproof ones, like the Xperia AceII

You're implying it's because of cost-savings, but OEMs still typically offer 3.5 mm jacks on their lower-end models (even waterproof ones, like the Xperia AceIII). It's the expensive flagships they've removed them from. I think a better explanation is that high-end models are fashion items while lower-end models are functional items, and it's fashionable to copy Apple.