The only reason why it should have realistic graphics is this:
Realistic graphics look like shit 2 years later. Artistic does not.
So by bringing out an art style-ish game like SkywardSword or Wind Waker earlier and then a few years later a realistic Zelda doing so will make the realistic Zelda look dated day1. (Twilight Princess looked OLD day1. Wind waker looks still better than TP)
So if they want to make a realistic and art style-ish Zelda they should do the realistic one first.
Making art style-ish games also helps hide the fact that hardware is dated after a while.