Tens of millions seem to disagree but thanks for your opinion.
Tens of millions would disagree with me here when I say that more than half of those millions of upgraders upgraded just because they could, not because they needed to. While the title with its blanket insinuation is certainly dramatic, OP's post is far more tempered and in context.
My iPhone 7 and 8 still run beautifully for what I use them for. If I am just having an itch, this is the kind of people I think OP is speaking to, with his 4 points. Unless I concur with any or all of those 4 points, there is no reason to upgrade, is what OP suggests. Which is fair of him to suggest, in fact, it is quite what it is. He has covered almost all the grounds on which an upgrade would be warranted. Justified is another matter, we can justify anything in the world.
The only thing I would add here is that even if my 7 worked perfectly for my needs next year as well, which knowing my usage it would, I would still consider upgrading if Apple stopped providing iOS upgrades for it. So, technically, while I would have zero reasons to upgrade for features, hardware and performance advancements, I would upgrade just to stay current with software. I might make it a year or even two without the OS upgrade, ride it out for the excellent security updates Apple still provides older phones that do not receive OS upgrades, but if the new software provides something that really want, that would make me upgrade. Which is what OP seems to suggest, that unless those 4 points are in consideration, there is no reason to.
Each person has their own reasons and considerations for upgrading. Mine would be:
- if software support ends, I would be tilted heavily towards upgrading
- I do not care for camera improvements at all
- I am satisfied with battery life of my 8, even if new device provides 16 days of life it would not matter to me as the only reason for upgrade. This would be a nicety as of now, with my current use and need.
- display is also fine on my 8. I do not want a better display today. When this phone dies or loses software support, I will then buy the best I can afford.
- speed is fine on my 8 for my use. Yes, the new devices will be faster, but I do not need that speed today. When I need a new device, that is when I will look to get the best I can afford for my requirements.