I think you're partially right. Some customers just care about the "headline" number of the power of their car, and between a 300hp lightweight SUV and a 450hp heavyweight one, a lot of people will go for the heavy one. This is because in car advertizing most of the time they stress the output.

BUT as always life is a pendulum. Late 60's early 70's, cars in North America were huge and were averaging 12-15mpg (18-23l/100km). Then the oil crisis came and car makers produced smaller cars up until early 80's. For the past 2 decades we have been upsizing and we are now back to this type of fuel efficiency with the Ford Excursion or the Cayenne Turbo... This may be a sign we are at the peak of the cycle.

I do agree, reducing weight isn't easy when you don't want to de-content the car.
The chassis is the main piece where we can gain weight. Audi is gaining basically 300kg (600lbs) on its A8 compared with the 7 series and the S Class. Body panels are also important with thinner but stronger sheets of steel or plastics. The engine and the gear box are also important elements.
Inside of the car there is also a lot to gain with seat frames, different type of plastics.... Unfortunately the trade-off most of the time is the noise.

Bottom line, I think the client isn't ready for the time being to pay a premium to get a lighter car because for most of the people the heavier the safer!!!!

I know at least 3 persons if I include myself. I would be more than happy to drive a light and luxurious SUV. It would have less inertia and would drive more like a go-kart....