> They improved the software so much on this iteration of iOS that they probably relieved a lot of the hardware upgrade pressure users were beginning to feel in the first half of 2018.

Another other side of this is that they've given people good reasons not to upgrade: I've been surprised by the number of people who commented that the new phones are uncomfortably large to hold and, of course, a fair number of people don't like losing the headphone jack. None of those are complete showstoppers but since costs have also gone up around 20% beyond the rate of inflation[1], you don't need to give people much of a reason to delay a purchase which isn't necessary since everyone knows the phone you buy next year will be even better.

1. A quick spot-check comparing the iPhone 6S & 6S Plus we bought in 2015 vs. the XS / XS Max now has it at 24% with no improvement in storage capacity.