There are plenty of waterproof phones with headphone jacks.
With that in mind removing the 3.5 jack does not make the phone sleeker you need to see the adapter as a part of the iPhone as the phone is mostly useless without it.
It's the exact same size as the 6s.
It may be a smart move in the end, but it's not obviously so at this point.
The headphone connector is already pressed close to breaking point in my pocket, I can't have something even bigger sticking out of the phone in a tight pocket.
Yes, another proprietary connector no better at it's task than the equivalent industry standard connector.
I lose headphones like crazy, and I'm not an audiophile, so my average spend on headphones is under $20
There are other similar models around the same price. You don't have to spend $100+ to get similar audio quality to $20 headphones that happen to have Bluetooth as well. Bluetooth chips are stupid cheap these days, pairing that with a small battery really doesn't add much to the cost of normal cheap headphone equipment and you'll find plenty of pairs <$40.