I think what you are all seeing is the spoiler that's attached the top of the tail gait. That spoiler is not the flat side of the tailgate. I remember touching it during our local meet and greet of the truck. It's to help air go over the tailgate.
Agree with many others the 4 door of anything is a compromise. People buy the four door versions based on the perception and legacy 2 door version created. Think wrangler. Historically the wrangler image was based on the 2 door not a 4 door - the sad compromise. No one wants a 4 door, they need...
I would say Slate is an example of how many people want a shift to a simplified platform. The Model S and X are the most extra-heavy-luxury models Tesla sells.