I agree. The flaw to Tesla's plan (to me at least) was it required the dealer to install it. It isn't as simple as putting it in the bed and connecting a simple harness.
For me, the design should be self-contained. The user should be able to install it (ceiling harness in garage) and it should have its own internal thermal management with a cooling fan (electronic trans cooler). It would make sense that in the summer the owner would need to keep the bed open so there is adequate cooling.
The problem would be, when not in use, and removed from the bed, how do you maintain it? There are people who would need that range extender enough that they would leave it in all the time, but I would assume this group is smaller than what would be needed to recoup R&D + manufacturing for them.
Since Tesla has access to SOOOO much user data, I bet they looked at current CT battery usage and saw how small this actual number would be. If I was in charge of the project, that is what I would do. Look at CT charging of existing customers and see how often users charge to >80% and drive until needing to charge again while a trailer is connected (trailer plug connected). I'll bet this was such a small subset of owners, just like most other 1/2 ton pickups...