I'm not so sure that the reason Twilight Princess sold so well was because of "realistic" artstyle, that might have been a small incentive for some gamers, but imo, the reason it sold so well was thanks to the Wii, more specifically, the Wii launch. Everyone and their mother bought a Wii during that period and Zelda was probably the "best" game during that time (outside of Wii Sports), so Zelda took the spot for a very long time. If Wind Waker and TP would have changed places then WW would be the second best selling Zelda game right now.
That said, is just a matter of circumstances, if the Wii U keeps selling like right now and the next Zelda has a realistic artstyle, then the game will probably sell the same as WW. If on the other hand the Wii U starts selling very well, then a realistic looking game might give the game a little boost, but is by far not the main reason people will buy or not buy the game.
The problem is that we really don't have any evidence to proof if a realistic Zelda game will really sell more than a "cartoony" one. Like I mentioned above, TP got extremely lucky, so is no base to proof this. Another "realistic" Zelda game that sold really well was, well of course, Ocarina of Time, but again that game had the benefit of word of mouth, it got the tittle of being the "best game of all time" for a reason, and that's something not many games have the luxury to have. We also have Majora's Mask, which had the same artstyle as OoT but it didn't come close to sell the same. WW simple had the problem of a very low install base, and Skyward Sword has the "problem" of needing an extra accessory to be able to play. So really when you think about it, is really hard to compare Zelda games because most of them released during very different circumstances, making comparisons a little bit hard.
Edit: Well we do indeed have 2 Zelda games that are easier to compare. Skyward Sword and Majora's Mask. Both games require an extra accessory (MM required the N64 expansion pack and SS required the Wii Motion Plus), and both launched pretty late in each console's life. One has "realistic" artstyle (at least at the time it had) and one has a "catoony" style, and well, according to VGC both sold practically the same: 3.69 for SS and 3.36 for MM. So this kinda proofs what I said above, the artsyle doesn't really matter all that much, it all depends on the circumstances.
Nintendo and PC gamer
