A similar argument could be made for 6s to 7 though (and I suspect a lot of people did make that argument), so there will be plenty of 6/6s people out there who will see it as an improvement.
From the circle of people I know, it's rare to upgrade every model. Every other model upgrades seem more common.
There was little perceptible difference in speed when navigating around iOS generally.
Heavier hitting apps loaded faster, but not noticeably so.
Slack behaved exactly the same, even scrolling through long histories.
I ended up getting an iPhone SE to save money ($175 credit for my iPhone 6 Plus helped). It's a 6s with a lesser camera, 4" screen. Runs smooth as butter.
From the circle of people I know, it's rare to upgrade every model. Every other model upgrades seem more common.