One of the reasons so many people are surprised at the current depreciation is that for several years, the model S, showed very little depreciation.
The prices of the cars were steadily rising, and new option packages were released that made current models more expensive than those purchased before.
People were able to buy a new Model S, take the $10,000 tax deduction, and then resell the cars for about what they paid for this.
This, in addition to the increased fuel savings (gas was much more expensive then...and rising fast) allowed many early adopters to drive almost free for a few years.
Things have now changed, especially with the release of the very well equipped 70D. Many previously high priced options (sensors, turn by turn GPS, folding mirrors, free supercharging etc) are now standard.
Believe things are just getting back to normal, compared to other premium cars.
Same thing use to be with Ferrari and other exotics. You could order a new one, and immediately sell it for a huge profit...no more.