Honestly, modern phones last 4-5+ years. The two-year upgrade cycle made more sense when the technology was rapidly improving and there were significant changes in user experience from year to year.
The only thing is you may have to replace your battery halfway through the life-cycle of your phone, but that's not a big deal.
I genuinely don't get the appeal of the Fairphone. Just keep your normal smartphone (be it Android, iPhone, or whatever) as long as you can then recycle it once you're ready to upgrade.
Which major smart phones over the last ten years have not had replaceable batteries?
Even if the iPhone battery can only be replaced by an Apple-certified repair shop, you can just calculate that added tax into the overall price of using the phone over 5-6 years. I don't really see what the issue is. It's not like it's expensive to have your battery replaced, and you only need to do it literally once over the useful lifespan of any phone.