Right, if camera resale value is sinking fast it means that there is rapid improvement in the new bodies, taking away from the value of the older models.
If used equipment resale value remains high then it means development of new tech is no longer fast.
In my opinion it would be normal for a person to buy a camera once and use it until it wears out. That would be the best from an ecological point of view, and probably for the buyer's economy as well.