These phones don't exactly hold "value" past the first year. As all technology does, over time "lose" value. You may as well sell it for as much as you can each year, and get a new one. If you can get $500 for one of these phones, and you may only owe $300 (just an example). You just made $200 back. Now, if your really hell bent on keeping your phone for 4 years. Go right ahead. But, it will function slower over that period of time. The battery will not hold a charge as well as it used to. You will miss out on any new features that have come out over that 4 year period of time, etc. Just cause it is supported doesn't mean you get anything new other then the OS, and ability to run current software (to a point).
To me, it makes no sense to hold on to old phones. They do so much, and updates make them do more. Why keep something that simply can't do it as well as the new one? Especially, if you can cut down on the cost of a new one by selling the old one?