Well, batteries tend to only last a couple of years because batteries suck. The display you can probably keep for a good long while, but the CPU/GPU combo will be out of date and insufficient for modern apps. I ran into this with my original iPhone (2G model) which basically ran out of steam after a couple of years. Cameras seem to get better with each iteration, and I certainly wouldn’t mind improved low light performance for instance. Bluetooth and wifi are pretty much unchanged, but those are a relatively tiny part of the phone. Cell technologies keep moving on as well. The iPhone has already gone through two generational shifts in cell technology (EDGE to 3G, 3G to LTE) in the short time it has been around. Storage tends to just fill over time as you buy more music and apps and take more photos or videos.
