Too many posts to read back and I suspect someone might already have said this but here's my take anyway...
I wonder whether the XS is being more impacted by the wait-to-see-the-XR effect than the XS Max is.
I was never going to go with the Max because I have quite small hands and I was pretty confident that my 6s was already towards the size and weight limit I would tolerate but even for me I very nearly waited for the XR to see if maybe I could push things just a bit further and the XR would be the limit on the size and weight that I was willing to carry around with me rather than the XS. For people who were already happy Plus users or who have already decided that they want the biggest screen that Apple can offer them on a phone and are willing to pay for it then there is no reason to wait to have a look at the XR, they might as well get the gratification of buying the XS Max straight away.
If it hadn't been for the fact that I am leaving for 3 months of travels on 17th October I probably would actually have waited to have a look at the XR before updating my 6S but I got impatient and didn't want to leave it until I got back in January to upgrade so I have gone out and bought an XS but even I, with my teeny-tiny hands and preference for smaller phones ("smaller" is all relative of course), was very nearly one of that wait-to-see-the-XR group that I'm hypothesising about.