The problem with the Fairphone,beside the price point, is the fact that it is not state-of-the-art now and that will be annoying in the future.
The mediocre Battery life and the decision not to include wireless charging is one of these points, the camera,etc. as well.
I really looked forward to the FP5 but decided against it for these reasons.
Would that really help? I remember that each brand had their own custom battery sizes, and it would only sell the remaining stock once the phone was no longer on the store shelf. I only know Nokia had some standard form factor for batteries.
Batteries were phone specific but it wasn't a big problem to find them. I bought several for my Note 3 and it allowed me to use the phone for a long time.