In all honestly, I think aftermarket car software retains its value better than physical aftermarket parts.
If you add a spoiler to the car, that spoiler will take some wear and tear and be valued by the car shopper at a price lower than what was paid to install it in the first place.
But if you add a piece of software (software unlocked physical features, FSD), the software itself doesn't degrade. And if the second owner wants the features unlocked by that software, they're either going to need to find a used vehicle that has it already, or pay Tesla the exact same amount of money that the first owner would have paid to unlock it.
Used Teslas with FSD are valued higher by roughly the entire price of FSD. And I bet used SR+ with software unlocked premium interiors will be valued higher by roughly the entire price of unlocking the premium interior.