I found carrier's promotions to be superior that what was coming from Apple. I think if you're not all that interested in an annual trade in, you might be better off money wise taking advantage of other deals.
I was never really tempted by apple's IUP simply because I don't upgrade every year. I think the X vs. Xs is the a great example of this. The difference between those two models is extremely minor and not really worth the effort imo. Others may disagree but that's my opinion of the Xs.
I found the same re carrier promotions in a recent investigation. ATT offered me $450 off any phone including the XS and XS Max (and the XR once released... offer not good for pre-order) to upgrade among our 4 lines, but, that line would be tied to a 2 year contract (and must be paid up front). I don't like the idea of the contract. But OTOH we've been with ATT for so long it hasn't really mattered. Phones ARE locked in that offer, but, we now use the $10 Intl Day Pass rather than swap in a different SIM when we travel. So, locked = not a big deal. And, we can upgrade (again) in a year with ATT if we start a new 2 year contract at that time.
Still leaning toward the IUP if I decide to get a new phone however. The flexibility appeals to me, as do the monthly rather than lump sum payments.
As to the second thing I bolded, I'm less sure. I think overall, this is definitely true. Incremental speed differences and other small things are not worth the trade. However, the camera in the XS is a BIG improvement IMO over the X. Even the XRs single lens example is better in certain respects. The larger sensor is a very big deal and produces noticeably better photos. I've compared side by side as have many reviewers and commenters here. I hate to say night and day, but, a very big and noticeable difference. For me, that might be worth the trade.
I've been focusing on this because I've had reason of late to scroll through our photo library. We have digital camera photos (2MP!) dating back to 1999. The photos get better and better every year/camera as we traded to better point and shoots and even a DSLR at one point. Starting in late 2007, our library begins to contain iPhone photos. Compared to the later point and shoots we had, and of course the DSLR, the photo quality is embarrassingly bad from the iPhones.
In the ensuing 10 years of course, the scales tipped. iPhone photos got better over time; so much so that we used the DSLR less and less, and today hardly at all. Still, the X's photos are not as good as what we get with the DSLR. So, an even better iPhone camera is something I would really like and appreciate.
Understood that that is not a consideration for everyone.
