there’s virtually no visible difference between iphone 6 and 8. Apart from dropping the jack and getting a waterproof design in return, which will increase the probable lifespan of the device if anything. So why upgrade?
While I agree there isn't a huge difference any more, the iPhone 6s is noticeably faster than a 6. The gap from the 6 to the 8 is presumably even larger.
I find it kind of depressing* , but people do use their phones constantly, so things like performance do make a difference.
* Yes, ok, I've been a mobile app developer most of my working life, so I shouldn't really complain.
