So ignoring the fact the iphone 6 and iphone 6s were best sellers and a myriad of new functionality on both hardware and software has been added yearly, you think the iphones have stagnated because a few emojis and wallpapers have been added? Interesting. It may be your opinion but it doesn't jive with the differences between the iphone 6 and iphone 7.
iPhone 1 brought the world a whole new device, the smartphone. A staggering innovation compared with the awful implementations from Nokia and Microsoft
iPhone 2 (or 3 as it was called) moved this forward and added 3G. Incremental, but a massive improvement on the iPhone1
iPhone3s - incremental
iPhone4 brought the retina screen an wonderful innovation and massively updated the look and feel
iPhone4s - incremental
iPhone5 brought not-a-lot except for the screen size change and lightning connectors
iPhone5s - incremental
iPhone6 brought the excellent fingerprint reader. Brilliant innovation and very well executed. IMHO the best phone to date
iPhone6s brought "incremental" improvements or not-a-lot
iPhone7 brought waterproofing which is a good feature although available on other phones for some time (Apple playing catch-up). It also lost the universal phone socket which means adapters need to be used for existing peripherals, a massive sub-optimal step backwards. A dealbreaker for many, definitely for me.
I've owned all of the major updates - 1, 3, 4, 5 and 6. I will definitely not be buying an iPhone7. If I had to replace my phone I would consider buying out of the Apple ecosystem - would be very expensive as I've some specialist applications which I use.
But hey, I'm just one person and everyone else is happy.