the current 6 cly is already at its limit design wise. its already a buzzy engine and relatively unrefined NVH wise and mileage wise than its competitiors. They cannot add more displacement to make it more powerful and twin turbos will increase cost and complexity and bring it to close to the gtr SO
If they have a reshly designed 2 to 2.5 l 4 banger with a turbo they can have more torque and about the same power. Add DI to boot and they can have better fuel consumption due to the higher CR also. Meanwhile the car gets lighter and maybe the price is maintained. you see a similar trend in BMW and the replacement engines are sucessful.