Sure, there might be a few people who always want "bigger with more power".
I guess they can always add a supercharger or a turbo to the huge 6.2 Liter V8.
(Although Lincoln decided to go "all V6" for the 2015 Navigator).
But a powerful Turbo V6 Diesel does make more sense.
Not sure if GM does have one though. Or even working on it. As I have never heard about a new V6 Diesel from GM...