And the rotory applies to the G8 replacing the GP how?

GM learned from caddy that you can't have two of the same size cars selling in the same brand. Just look at the abysmal sales of the STS.

Also don't forget the G8 also replaced the Bonniville. Sales were falling and Pontiac was trying to slim down the number of models. The G8 was a good compromise between the midsize GP and the full size Bonnie.