Your post make a lot of sense on its own. I will add a few other factors:
1. Bigger trucks have lower gas mileage so converting them has a bigger impact on corporate mpg.
2. Competition is focusing on larger trucks.
3. Most large trucks are only occasionally used for large loads or hiigh mileage.
4. The Lightening has the capability of supplying power at the worksite with its large battery.
You make good points. I guess to your point # 3 is that some people get a large pickup as a status symbol and not for practical use.