I upgraded every year since the 3gs, but there was always reasons for me to upgrade until the 7.
Same situation here. I upgraded yearly ever since the iPhone 4. Last year was the first time I didn't. I had a 6S which I loved, but wanted to move over to the Plus sizes for the battery life, better camera and bigger screen, so I pre-ordered the Jet Black 7 Plus, and even though I was there right at 3AM EST it was still sold out and backordered for six weeks.
That gave me some time to think about it, and I ended up getting a 6S Plus (used) for half the price of a new 7 Plus, and available right away instead of waiting six weeks.
Turns out I absolutely despise the Plus size. Yes, the camera, battery life and screen size are better, but for my hands, the phone is nearly unusable with how huge and clunky it is. It barely fits in my jeans pockets and I have to put it in a case just to be able to handle its unwieldy body without it slipping out of my hands (I usually left my iPhones caseless).
I've been toughing it out for an entire year waiting for the next phone and reasoning that it would be an even bigger upgrade for me since I'd be moving up two generations, but this iPhone X fiasco has me very conflicted. If they had just released the 8 and 8 Plus, it would be a no-brainer - I'd have pre-ordered an iPhone 8 in a heartbeat and even been happy with the new features, which I consider a significant upgrade (wireless charging, faster chip, etc.) but knowing that the iPhone X is coming out a month later makes the 8 already look long in the tooth.
The other problem is the fact that the X isn't coming out until November, and knowing how bad Apple is at matching supply and demand, it'll likely be sold out until January...which means I'd have to keep my hated 6S Plus for another 5-6 months.
Now I'm leaning towards doing what I did last year and just picking up a used iPhone 7.
In a year when Apple should have been getting even more money from me than ever, now they probably won't be getting anything, two years in a row, because of their inability to competently manage their supply chain.