iPod touch is thinner than an iPhone and still has a 3.5mm jack. Thickness considerations isn’t a reason to remove it. The thickness of iPhone 6S/Plus actually increased compared to 6/Plus to accommodate the, to me, gimmicky 3D touch display assembly. The size of the battery shrank, too. So, if Apple really cared about thinness and battery life, they wouldn’t have included/will remove the 3D touch stuff in the next iPhone. I think controlling all aspects of the device (perhaps a DRM move?) is one reason but I have yet to hear a compelling reason to remove.

I suspect that if they were to remove it, they’ll do it without trying to justify it at all. They’ll present it as the way it is. See the App Store search results Ad. announcement, which is borderline tone-deaf, for one recent example of doing something that wholly benefits Apple and no one else.