This. Updates like force touch, better cameras, portrait mode, faster processors to handle the increased demands of iOS, etc. Not to mention people moving to bigger screens.
Have they been major updates? No, but the incremental updates have been enough. And for someone with an aging phone (like my 6s), you're ready for a new one. Could probably just put a new battery in and be fine but I'm looking forward to a bigger screen, better camera, and portrait mode.
How is the battery in your 6S? On my 5C I would have it die around 18% charge remaining so I knew it was time for a new battery but I was nervous at first to leave it overnight because I could not get an official Apple logo'd battery, it was made in China. It would get pretty hot the first couple weeks but then seemed to settle. The electronics repair place told me that even the Apple original batteries are made in China however I have read that some aftermarket batteries may have thinner walls on the battery housing and there can be issues over rupture or leakage eventually. This is why Apple probably carefully hand picks what will go into all their devices to avoid something like the Note 5 fiasco.
Still for the last 6 months that I had it with the new battery it was great, but then the processor really started to show its age, and it becomes a test of patience to perform basic tasks.
My iPhone batteries get a workout because I have to charge 2/3 times a day and I am hoping the 7 will go the full 2 years that my contract is in effect.