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.
Security updates will end 01 Sep 2028.
Fairphone 5 and earlier have end-of-life Linux kernel branches without security support. Fairphone's more recent devices are headed to the same situation. In practice, the same thing happens with other components beyond the Linux kernel.
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.