Very, very interesting. I love articles and studies like these - thank you so much for sharing it!
This could mean a variety of things. Perhaps phones are getting too expensive and so people are hanging onto their already-expensive-and-purchased phones for longer periods of time.
Or, maybe the phones that came out in 2017 weren't enough for some to make the switch from their current devices. Or maybe they wanted to see what upcoming iterations offered (would they be better than 2017's models; thus encouraging a switch?) before they gave up their current devices.
Or, perhaps most customers (myself included) decided that midrange was the way to go in 2017. The midrange market is exploding now and in 2017 and even earlier, people may have decided that midrange phones had just as good a performance as flagships; or some midrange phones had specs that were just as good, and so they bought products like Honor, Xiaomi, OnePlus, etc. Maybe they did this for economic reasons; or maybe because they were enticed by the midrange brands.
I remember the OnePlus 3T (though released in 2016) was a huge hit. Perhaps people held onto phones like these because they "worked" and therefore didn't see a need to upgrade in 2017. Going off of this, I got my Honor 8 (a midrange phone) in 2016 and though a lot of good phones came out in 2017, I couldn't in good conscience justify a switch so soon after getting a phone that "just worked" for me and had no issues.
A lot more can be said about this specific variable. It could be an entire study in itself.
Again, very good thread topic!