Side by side, the 8 is impressively faster than the 7. I don't know about cameras or anything, but the CPU upgrade was no joke.
I've got a 7+ and frankly I'm struggling to see what more I could want. So probably going to skip a generation.
I reckon it is topping out though. A bit like any i3 or whatever is now sufficient to do any office task imaginable. You're not going to spreadsheet any faster with a i7...
Also, anyone who's moving up from a 6 or 6S will see a huge speed improvement if they move to a 7, and for most "normal" usage I'm not sure the move to the 8 would make that much difference.
The issue is that it's not enough to be good enough 99% of the time. That's assumed.
It's performance in the last 1% that really makes or breaks the user experience (or maybe the right ratio is 95/5 or 80/2...).
Obviously, any car will have sufficient power and traction to maintain cruise on the highway. It's those few seconds of acceleration to pass, or that panicked emergency stop, where you'll definitely notice a difference.
But your point stands that 100% of hard core gamers that want the biggest fastest new thing have 0% use for the 8. (The x/ten has the bigger better screen too)