I agree that Tesla's strategy on trade-ins is difficult to figure out. At various times in the past I asked for trade-in values on my Roadster and both of our Model S, and all four times the amount they offered was ridiculously low.
However, they talked me in to upgrading my 3 RWD to a P3D by making a very high offer (enough that, financially speaking, the 3 RWD never happened). It only had 2k miles whereas the other cars had far more, so perhaps that is the issue - they only want to re-sell low mile cars?
But even if so, why do they often offer less than they can get just re-selling to somebody like CarMax? The only explanation that makes sense to me is...because they can. Perhaps enough people take them up on the deal that they come out better financially than if they offered more, and got more people like you and I to trade in? I don't have the numbers to say this is the case; it's just a guess.
Plus, while owners upgrading is obviously a great thing for Tesla, it is probably less important that an existing owner upgrade now (heck, we know they'll upgrade later, right?) than to get an existing ICE driver in to the brand.
