The battery on the MS is half the cost of the car. In 5 years, I think it is safe to assume that battery pack cost could be half of what it is today. Used MS's value will decrease faster because it will be the only case where the new car is very possibly cheaper then what was paid in the past for the same functionality. Picture 5 years down the road with a MS with 265 mile range for 50 grand (when everyone paid 70 at the start), used cars will take a hit. You see examples of this with the Leaf when they dropped the prices this year where used leaf's with 10,000 miles are selling for 13,000 bucks.
Yes software updates might be cool, but you are still dependent on the physical devices present on the car during the time of purchase. You will never get an update for self driving as the sensors to make that happen will not be installed on your car. You may get better navigation, system sleep functionality and so forth which will incrementally increase the value of used cars but there will always be certain features in the newer model that cant be supported in the old models. For instance Telsa is planning on implementing 4G (at least that's what i heard), but the hardware to make that happen doesnt exist in the early cars.