Well if you recommend getting an older phone because it's cheaper, GrapheneOS support may be a concern. Also I think a phone usually can last for 7 years with 1 battery replacement, good ambient temperature and careful use.
I mean, the 6a still has 3 years of support left so whilst it is older it is hardly at the end of its supported life.
Not everyone can change a battery in a phone, I can but I would still rather not do it on a phone that isn't really anything special and whilst yes they could feasibly last that long I think in practical everyday use application by the time you are getting to three years of daily use it will be beaten up and physically not in great shape any more for your average user.
3 years is not that much unless the user doesn't mind changing phones rather often and beating up a phone in such a short time is just a massive skill issue tbh.
My pixel 6 is about 3 years old and the only wear I can see on it is a single little micro scratch in the top right corner of the screen that I can't see without a light reflecting off of it. I don't bother with a screen protector, just a thin silicon case. Battery is fine for about 2 days of normal use even though I regularly use a wireless charger.
Whatever idea you have to phones, you‘re wrong. They can easily make 5 plus years if you treat them right. The more problematic part is daily use and battery degradation/repair.
But google sucks anyway so I‘ll stay with postmarketOS on my oneplus6 and wait for my camera to come to life some day (hopefully).