Let's assume some measurable number of people might do this, purely for arguments sake. Why does the resale value of old iPhones not reflect these devices being "unusable?"
You keep saying unusable - nobody said throttling makes it unusable, just slower than a year ago when you bought it. Value of iPhones is irrespective of the Apple admitted fact that they do in fact throttle older phones. Some one year later and some two but it is a known fact that it happens. What it has to do with resale values is a separate thing that would be interesting to debate if Apple had not verified the throttling part.