The biggest reason I swap phones every couple years is I feel like software updates (likely intentionally, at least by Apple) turn older phones into a slog. Especially with the way contracts are structured now for upgrades, I don't see any reason to stay in a phone for much longer than that. Even if you buy instead of lease, as someone made the point in the 8 thread, when the new ones come out, the resale market is so aggressive you usually can get 60-70% of your value back, especially if you get it unlocked.
I'm not super concerned about my battery, my phone is only a year old and I see it get glitchy and have power issues every time there's a new update coming. I was just curious if I was imagining things, if it was my cord, etc. I don't ever really intend to keep a phone long enough for a 40-80 type regiment to be relevant. It won't be truly 'obsolete' in a technological sense (though software issues arise with processing), but if I can trade it in and get the newest model every two years, it's obsolete in my view. I don't even use my phone for work and I can't imagine trying to stay within the boundaries that article cites. I basically have anxiety set in if I wake up in the morning and it's not at 100%.