Everyone has their own needs and budgets which determine what they buy. No one would disagree that a V8 Ultimate is the car to own if you want the greatest Genesis experience but not every wants or cares about that.
The V8 is great for effortless power and I'm sure there are some tangible benefits of the adaptive suspension (US V8 Ultimate). The V6 is for people who need good power and want to keep their TCO low while still getting everything else great about the Genesis.
TCO, using US V6/V8 Utlimate RWD retail pricing, 60,000 mile span, and EPA combined fuel economy. I'll use $3/gal as an average since there is a huge range depending on market and grade over time. I'll assume all other things equal (tires, oil, brakes, etc...) even though they will be more for the V8.
V6 Ultimate: $49,950
Fuel: 60,000 miles/22 mpg = 2727 gals * 3 = $8181
V8 Ultimate: $55,700
Fuel: 60,000 miles/18 mpg = 3333 gals * 3 = $10,000
That works out to $7500 more and while it may be worth it to some, it wasn't for me. H3ll, I got a Signature so my TCO (before negotiation is nearly $15K to a V8U). The V8 is awesome but you pay a premium for it and for those who don't live in their cars or care about maxxing, it's a tough sell. Just like the folks who would not buy a Genesis but would happily go to a German dealership and get nickel and dimed for $10K more and still get less car for the money. Different strokes.