Happy FairPhone 4 user here. Running 3+ years, only replaced the battery once. Aim to keep it 7-10 years, barring unforeseen events like theft. Not going to replace it just because a newer model is released.
I've been running mine for ~4ish I think, on my third battery and might run out my fourth because it's feeling a _little_ sluggish at times.
I am waiting for the next FP model where I can use a USB-C/dp external display and then I'm all over it.
Someone stole my fairphone 4, then threw it on the ground. So it seems like i'll keep it a while longer