Even if a more realistic art style would increase sales numbers, then that just proves that Nintendo's top priority is were it should be: At bringing out what they think goes best with the series!
Seriously i don't care if WW sales could have been better with another art style. I like it very much and am glad they decided to go that way.
And i was one of those people who originally didn't agree to the WW art style, but after playing the game and seeing all the different expressions they drew on link's face and all the other details in the game, i learned that a different art style doesn't mean losing what was good in the previous one, but gaining something great instead of "just" seeing what you have already seen before.
For the new Zelda U i'm very excited about the game and like the art style so far even if it was just a trailer. I don't want to see the same art style again and again, i like that they have variations, and i believe, that even if they could sell more copies of a single Zelda game with a certain art style, they sure sell more in the long run over the series when delivering what looks great for this specific entry in the series.