That's one part. The other is that iOS 12 really did focus on speed improvements. For many (most?) people, an iPhone 7 with a new battery is still a great phone.
I replaced an iPhone 6 or 6s with a 7. I also find $1,000 just obscene for a phone. I'm a person could have afforded the OLED versions and chose not to.
And ios 11 may have shifted sales forward. I was still doing ok on ios 10 on my iphone 6. Ios 11 slowed it down (even with a replaced battery) so I got an iphone 8.
Now on ios 12 my iphone 6 is fast again. But, I probably would have bought the iphone 8 by now anyway, just because of camera quality, etc. The speed trouble just pulled my purchase up by a year. (I even bought a stopgap SE in the summer of 2017)
I suspect others also upgraded early in 2011 due to ios 11. Of course this situation is better long run now that its fixed. I'm so pleased with ios 12.