In this case, form is function--part of the function of a mobile phone is to be portable. I'm not saying that they shouldn't have been more careful with it--supposedly the 6s is less bendable, which means they could design it to be strong AND skinny, if they want--but part of what I want in my phone is for it to be as invisible to me as possible when I'm not actually carrying it.
Whether or not that meets your goal for 'function' is another question. I can easily see other people wanting a phone that's considerably more robust. With the thinnest case I could find, I've had my 6 for two years, dropped it many times and not had any breakage problems. (I do have a faint blue streak in the middle of my screen which Apple tells me may be due to excessive pressure on that spot, but as its cosmetic and not a functional problem, they won't do anything about it.)
We don't all have the same functionality goals. It's part of your job as a consumer to decide if your idea of function lines up with the manufacturer's.
(It's worth noting that sometimes form is exactly the function--jewellery and other fashion items and ornaments are a good example of this. Do not denigrate people that carry phones for a reason that also has to do with fashion; for them, that's ALSO part of its function. It's not up to anyone else to decide if that's a worthwhile use or not. Even nerds carry things for 'fashion' reasons--sometimes we pick something that's objectively the ugliest, bulkiest, whateverest item to telegraph that we're the nerdiest kid on the block to all the other nerds. There's nothing wrong with that either.)