Edit: Original response was a little tongue-in-cheek. More seriously though perhaps they will start bundling Bluetooth headphones as standard. A slim phone plus a "bulky" lightning to 3.5mm adapter doesn't sound like something Apple would do.
Because it takes up lots of space inside the phone? In a phone that thin, the space for a jack connector and its supporting circuitry might compete with another 50-100mAh battery capacity. That's a difficult position for something as obscure as the 2.5mm jack.
Edit: Original response was a little tongue-in-cheek. More seriously though perhaps they will start bundling Bluetooth headphones as standard. A slim phone plus a "bulky" lightning to 3.5mm adapter doesn't sound like something Apple would do.