The Taptic Engine existed because they decided to do away with the mechanical home button, ostensibly for "waterproofing." The decision to retain tactility led to necessity for a larger more precise vibration motor, and then punting the headphone jack out of the phone became the obvious thing to do, with a bunch of post-rationalization externally and internal justification hinged around accessory sales.

Waterproofing and reliability -- word started to go around that the home button would break after a while. The "fix" for this was to make the home button a fake capacitive button, and that required a bigger vibration motor.

AIUI the home button was by far the least reliable component left on the phone and the biggest driver of AppleCare support for non-accidental damage.

The problem was so bad that it became an Asian cultural thing to turn on the accessibility feature that gave you an on-screen home button control and to use that exclusively, literally never touching your real home button, because you needed the real home button to remain intact for when you wanted to sell your phone later.

Another option would've been to remove the physical button entirely, which is exactly what happened with the iPhone X (I think? I've never used one; only seen pictures) and with pretty much every flagship (and even less-than-flagship) Android phone nowadays.

So they did create space by removing the headphone jack; you just think they used that space unwisely (or that that space would have been best used for a headphone jack).

The 6S had a taptic engine AND a 3.5mm jack.

And ~100 fewer mAh than the models mentioned above, which precede and follow it. Seems like Apple did use the space to increase battery size, then.

I wonder what the net change in weight and volume was with the removal of the home button and expansion of the display.

Maybe? Would have to look at side by side of 6S and 8. The 8 is a tad thicker, but that may be the glass.