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.

The 6S Plus was $750 for the most basic model at launch and now ebays for about $200 which squares with losing about half the value per year which is pretty normal.

Consider also the fact that the battery life will be substantially decreased from new and that most people wont be able to replace it themselves.

Something that is overprice is actually more likely to decline because 2 years from now its just a comparatively slow device that no longer serves as a status symbol.

It is only true because there is a second hand market that trust this is the correct price. Seeing how those devices need to move technically with their time (more powerful hardware), I fail to see the attraction of a device that would be well passed its prime, specially with a battery that isn't replaceable.