I upgraded for a few smaller reasons. The ability to use Hey Siri at any point is actually quite nice if you have HomeKit-enabled home automation devices. It's also nice for me when I'm doing work in the garage or the kitchen and my hands are dirty. I tend to listen to music, and if I want to change the song, the last thing that I want to do is get that gunk all over my phone.
3D Touch sounds interesting, and I'd like to see if it can make my workflow more effective. I don't think it'll be as helpful as TouchID, but we'll see. 4K video could be fun, but I think I'll gain more out of the higher resolution when dealing with zooming at lower resolutions.
The only negative thing is that while I'm gaining 4K video capability, I'm losing some low-light capability as well.
Ultimately, an upgrade should make your phone experience better. For example, I went from the 5 to the 5S for TouchID, and I was quite pleased with that.
I have the iphone 6. Its paid off. I guess after I trade in the old phone the balance would be about $480(estimated) $20 per month for 24 months on Tmobile.
If you want the most value out of your current phone when upgrading early, you should avoid the easy trade-in route. Trading in a newer phone is rarely a good deal for a consumer. For example, I could trade in my 6S+ 128GB for ~$400... or sell it for $600-700.