My reasons have changed over the years, for example, the camera used to be one of the top functions I looked for with the phone updates.
I have recently gotten more in to taking better photos and have 2 dedicated decent cameras that I keep with with a lot of the time. I noticed myself taking far less photos with my fun and with that noticed that the idea of updating to the very next phone has lessened.
The X for me is already a very good looking and capable phone.
I like the OLED, I like the size, I like the asthetics, the speed.
I have upgraded every single year on day one release from the initial iPhone and the 3GS is the only iPhone I skipped.
Starting with the 6Plus I started maxxing out memory to the highest available.
Having all of the Plus phones and now back to the X size. I just like that weight and form factor so much better, I don't think Apple will pull me back to a phablet sized phone, at least not for this next upgrade cycle.
So now, iOS, features, display take top priority.
Also, since my preferences ahve shifted, I can potentially take that $1,000-$1,200 upgrade cost and put it towards new camera equipment.