I agree with Random Person here. Nintendo chooses a distinct artstyle for each game depending on the specific emotion or message they wish to project. Skyward Sword was meant to portray a bright and vibrant world prior to the appearance of Ghirahim and Demise and it did so very well. When darker scenes needed to be portrayed, however, especially starting from the Ghirahim bridge cutscene following completion of the Lanayru Mining Facility, Skyward Sword executed so very well. Given the history of Nintendo reusing most franchise artstyles (same on in LA and OoX, same style in both N64 games with slightly updated engine in MM, cell-shaded graphics in TWW and DS sequels), there is a high probability we may see Skyward Sword's artstyle yet again.