I'll admit to being one of those who claim they won't upgrade, but then do it anyway. However, I almost didn't get the X; I bought it when it first came out and returned it because of the price, despite loving the phone. Since I had the phone, when I found it in February this year at $200 off, I thought hard about how I wouldn't upgrade in the fall if I bought it again.
The reasons I bought the iPhone X again came down in large part to the "unique" design of this phone, with the beautiful display and the stainless steel body. My guess back then (which I still somewhat feel) is that the stainless steel would be dropped in the next iPhone, that they'll go back to aluminum, to bring the price back down more in line with other flagships.
So, for me, what I thing I'd need is that the new iPhone will still be made out of Stainless Steel and, secondly, that it will need to have a Samsung OLED panel, or look at least as good. I think the fact that the new iPhone will have an LG panel will keep me from buying the new iPhone. I think Apple will do a great job on the color calibration, as they always do, and even making sure they displays have strong quality control. But you have to believe they'll still have a similar "blue shift" when viewing at angles, which LG displays are criticized for; that as good as Apple will make the display, it just won't quite be as good as the iPhone X display.