Developing a DCT gearbox to support the torque of these twin turbo V8's moving 4500+ pound cars is difficult. Not to mention there needs to be some headroom as the companies know people will tune and launch them.
Audi did the same thing with the new S5 model and its turbo powerplant. They ditched the dual clutch transmission for a ZF automatic.
Who is the winner in this? ZF. They get to sell more transmissions and no doubt the cost factored into BMW's decision.
The next generation mid-size battle is shaping up as the most boring of all time. Audi, Mercedes, and BMW all have the same exact thing. Porsche has a similar 4.0 liter V8 motor as the other three but at least gives you a dual clutch transmission with all wheel drive which stands out in the segment.
Cadillac, please do something exciting with your next car that doesn't copy the already boring German recipe.