Yes but the reason for SUV's being the most profitable vehicles for automakers (actually, big trucks are if you make them) is not applicable here. It has to do with basically making them off your existing car platform (scale/leverage) but charging more. I do agree with mercuryrising though that this is the next extension in the original vision.

The Model X shares 60% of its parts with the Model S. I believe that, critically, the "sled" (battery tray and motors) are shared between the two.

Right. But no car they make is yet at much in the way of economies of scale due to their tiny volumes compared to a "normal" car maker.

Maybe they're doing it the opposite way? Designing the platform for the gen3 by making the model x, thus making the gen3 cheaper.