My iphone 6 works perfectly fine, and i'm almost guaranteed to lose one of the wireless headsets. The iphone speaker improvement is useless to me as I will use actual speakers if I want to listen to music outloud.
Bluetooth headphones aren't as good sound quality as wired so until they add a jack to their iphone 9 or whatever version it is when I need a new phone I won't be upgrading.
Don't really see why anybody would want to upgrade unless their job required overseas calls or that was something you did regularly. The camera on iphone 6 is still good, if you want to be a photographer then you'd use a real camera. Waterproof is only useful to people that somehow frequently drop their phone in water, faster performance I can't see the need for unless all you do with your phone is play games. Better brightness- not many people use full brightness anyway so kind of irrelevant to me and the stereo system- Again I don't really know when people use their phone volume above halfway unless you are showing someone a video