Did some math:
1. If I’d bought an iPhone 6 64 GB last year, I now could sell it around €600. The new iPhone 6S will be around €800 gross (these are Hungarian retail prices, I could source it from SF which yields around €630). Roughly I’d be spending €200 on upgrades yearly, but I couldn’t keep my old device.
2. If I bought an iPhone 6S for €800 and I kept it for 3 years, that would get me a yearly a 800/3 = €266 spend, but I could keep the old one either as a backup/test device or circulate within the family. 800/2 = €400 per annum, but 800/4 = €200.
3. I also could buy a used and cheap 5S for €300 and use it for 4 years, until it breaks. But where is the fun in that?