Matters on the definition of en masse. As anecdotal evidence many of my friends hold an older version of iPhone and refuse to purchase a newer model. I think 6s is the peak of mobile phones and for me there is nothing new that Apple could add. If they were keep producing 6s with better CPU or more memory over time I would keep purchasing that phone. Since they degraded the UX for me in the newer iPhones I won't buy it any more.

There is no definition of “en masse” based on “anecdotal evidence”.

All of the evidence from Apples financial statements when the X was introduced is that people did upgrade.

But to your point, the iPhone 6s was when phones got good “enough”, it is still fast and the only reason I upgraded to the 8 plus was for the larger battery and screen.