I mean.. that is (on average) roughly 2.5 years old. If your phone was expensive and is still fairly new, then it's not going to be affected by the stuff that pertains old, mid-tier phones, before it gets even older than those.
That's not an Apple/iPhone property but I find it fascinating, that they are able to sell it as such.
And when you slot that against Android options, 3.5 years is a lot for a usable, secure life of a mobile device. I can safely assume I'll get five years of good, secure perf out of any Apple device from the last five years and another 2-3 years (at minimum) of tolerable performance, and that's pretty hard to argue with in this market.
Don't get me wrong, it's not enough and I'd like it to be better; I'd like all hardware reusability to be better. I'm pretty big on it; I still have an iPad 3 and a Nexus 7 in use as house kiosks (which sidesteps the security issue that phones necessarily have). But if I am maximizing useful life, buying Apple devices has been less fraught for most, if not all, of my adult life.
As you tell yourself, the iPhone 8 isn't its former self with its original iOS version.
Even if that weren't the case, a six-year-old iPhone 8 has out-survived the useful, secure life of, what, every Android device not made directly by Google? Hell, a 3.5-year-old iPhone 11 has out-survived the useful, secure life of the overwhelming majority of Android devices, too. And, further, given that A11->A13 was the most significant period of perf improvement and energy reductions (the A14/A15 are moderately faster but the tail certainly appears to be here), that bodes well for its continuing usefulness.
Apps or OS doesn't matter, they are interwined.
Not really, and definitely not with more modern ones. I have an iPhone 11 from 2019. It was released with iOS 13. It is now running iOS 16.1.1. It is as snappy as the day I bought it. I'm considering a phone upgrade, but that's because of the improved camera, not performance (or even battery life).
My old iPhone 8 (2017) is still getting updates, and now is moderately pokey with nontrivial apps, but the OS is fine. And I get it with regards to apps, as the perf and battery improvements between the A11 and the A13 chips was pretty significant.