£1500 *is* ludicrous, agreed. I already had this argument last year with friends who just *had to* have the X... Oh the first day. Despite complaining how expensive living in Los Angeles was.
However, I disagree with your depreciation argument. When did the iPhone6s come out? Have a look on eBay for a decent-condition iPhone6s and you'll find that their value remains still surprisingly high for a phone of its vintage. It's holdouts like me who probably keep the market alive! ;-)