That is the key. The phone is used so much it is worth it.
Let's run some basic numbers
With the cost of upgrading being about $300 bucks ($750 to buy new phone, sell old phone that was kept in case and is in great condition for $450) and the phone being possibly the most important piece of personal tech you own, upgrading really does make a lot of sense if you are U.S. middle class or better off. If you are on a tighter budget, then maybe saving the $300 this year makes sense. But by waiting another year to sell your 6 you also eat the depreciation of that phone's value during that year.
So how much do you save if you only wait one extra year? if you upgrade next year, then you sell your now one year older phone for $300, so cost next year is $450 to upgrade. So if you would have upgraded twice for total of $600 then waiting an extra year has really only saved you $150.