The art style is not completely Skyward Sword. There is a lot more detail and contrast, and the lighting is more TP/Windwaker HD.
Basically a blend of the last three, but with more SS.
That's what Zelda does best, a unique art style.
ANYWAY, OP is right, a dark Zelda would move more Wii-U's. BUT Zelda is not a system seller, Zelda is the most critically acclaimed series of all time. And we should be looking at how great this game will be (and it looks **cking great), not how many systems it will sell.