I upgraded from the 6 to the 6S and I like the latest and greatest so I will do so every year however in terms of upgrade its not been great.
Touch ID is faster so I now use that only 2 generations on as it never worked before.
3D Touch no need for this.
12mp and 4K not been on any holidays yet so never required yet.
Selfie flash never use this.
Live Photos used a few times but now turned it off as it messes my photos up.
Most of the S features are good to show off but you never use them much until a few generations later. E.g 4K may be popular once we all have 4K TVs and a larger entry model storage.
The 3D Touch is quite flawed as when you want to delete an app it thinks you are using force touch.
I never use Apple Pay either.
Processor wise the 6 was never that slow and I am sure it's still decent on ios9.
I feel this way with most S models. I've read 3D Touch is not poplar with everybody anyway.
Interestingly enough, this is how I felt about the iPhone 6 coming from a 5s; but for me it was mostly the fact that the iPhone 6 ended up being similarly specced (performance-wise) to the 5s yet I paid so much more for the new device, for what ended up being a mostly screen/size upgrade. I couldn't justify the difference in price I paid and I ended up with the first time I had ever regretted an iPhone purchase.
At least the 6s has (imo) justifiable hardware upgrades, with the significant processor gains over the A8 and the additional ram along with new features like 3D touch. The RAM makes power use absolutely wonderful. Feels like a brand new device. When I try using a 6 now, it feels "lacking" and if I try multitasking with it the same way I use my 6s, the differences are very noticeable.
That said, I can also understand why those who don't feel like "S" upgrades are worth the expense feel that way...I used to be in that camp. If all you do primarily is check your email, phone calls, texts and the odd internet search here and there, then there really isn't any point I don't think in going with a 6s when aftermarket iphone 6 phones are getting close to half the price if not 2/3's at the most. That can lead to significant savings, and you still get the advantage of a powerful device (A8 is no slouch even today) and you can still take advantage of a very good camera along with the bigger screen that is a big selling point for many people.
But OP, not to mean any offence, but the 6s is a very solid upgrade worth the difference in price between the 6 and 6s. I think its even more pronounced if you are going from a 6 plus to a 6s plus. There are more features the phone has beyond the ones you stated. Little things such as a more durable shell and stronger glass, better water resistance and significantly better front facing camera, and for future proofing the 2GB of RAM along with the 2x faster LTE and WiFi ac speeds over the 6 will see benefit a few years from now (though the LTE and Wifi speeds aren't noticeable today except in very, very small limited real world settings, when the faster Wifi and LTE becomes more of a standard the phone won't age as quickly as the 6 would...so if you're one of those people who buys a phone and hold on it for 2-3 years, it will age much better than the 6 in areas beyond processing power because the 6s has newer LTE and Wifi chips
capable of 2x faster speeds)
The 2GB of RAM makes a world of difference in work flow and multitasking. Ask anyone who was fed-up with the iPad Air and went to the Air 2; or the Mini 2/3 and went to Mini 4; or now the 6 & 6 plus to 6s & 6s plus. It makes a legitimate difference (in multitasking)
Ultimately though, I think those who feel like it was not a big upgrade and those who feel like it was indeed a big upgrade are both correct; everyone has different needs.
For some, processing power is a bigger incentive than a case redesign. For others, for their use, the extra power and RAM doesn't change how they use their phones. If all they want is a big screen, then the 6 is still a fantastic phone.
