Exactly, the residual value will be similar to most cars in 2018, but you could see a phenomenon where the used Model 3 will actually go up in value during later part of 2018 and early 2019 due to expiring tax credits and new model 3s will still be somewhat difficult to get. The used model 3s would be competing with model 3s that will not have a tax credit. This will be good for people looking to upgrade from a 3 to an S/X when they fall in love with EV but want more.
The other thing that will help Model S/3/X/Y retain some of the value compared to the competition like S/E/C/GL/ML Class MB, 3-7 Series BMW is the fact that you can upgrade your car through software. I initially thought it was a bad idea to include all the EAP/FSD hardware in every car, I am now rethinking that as it is now my contention that nearly every EAP/FSD like feature will eventually be activated by one of the owners in the line of succession. If the first owner doesnt opt for the features, maybe an autonomous taxi company buys the car at auction where bidding will be based on the value of the car to those looking to buy for example. This upgradeability gives your base model car a larger market. In theory, Tesla could also deactivate those features for the CPO market and allow new owners to reactivate them. Allowing Tesla more flexibility as it relates to the CPO inventory. They might lose a bit of money on the trade in, but would make it back with mark up once its activated by the next owner.
For me, I am keeping my X at least until early 2019 for this very reason. With FSD and tax credits expired, I expect the value to bump up and allow me to upgrade to the 2170 model S/X. Sure, maybe no one will want a none 2170 model X, but my guess is that many companies and people will be wanting an FSD car and it will very hard for people just discovering Tesla to get a model 3 early in 2019. By the end of 2019, I see the volumes better keeping up with demand, much like with S/X now.