Whether you see the value or not, there is a robust secondary market whose history shows that there is strong demand for used devices and that their entire value doesn't evaporate in a few years.

And if I'm wrong about 50% depreciation being typical over 2 years, I'm not wrong by much. There is not a huge difference between 70% depreciation and 50% in your monthly cost of ownership. Undamaged 64GB iPhone 6s are selling for between $230 and $310 on eBay, so that's 60-70% depreciation. On average the 6s cost about $20 per month.