That unfortunate day for the gaming world, when Link picks up a gun, or anyone else in the Legend of Zelda series for that matter, will truly be the day that the Legend of Zelda dies entirely for me. I've already voiced my dislike for the more advanced technology that we have seen in the past, such as the train from Spirit Tracks. I believe that the more advanced technology takes away from the game, and the series as a whole.
Let's imagine that Link did happen to find a gun in the middle of a temple, and decided to use it. How drastically do you think that would change the game? It would change the game entirely in my opinion, and that's something that I don't want to see happen to such a great series, especially a favourite series. It would just turn into a game in which you shoot everything once with a well aimed shot and they would be dead. Not to mention the fact that he genre of the series would change and become much easier, unless they gave it difficulty settings, which Nintendo wouldn't do.
I think that placing gun into the Zelda franchise would be a huge mistake by Nintendo, as I believe that many of the current fans would be greatly disappointed and forget about the series. I doubt that the addition of guns would attract anyway near the same figure of gamers that would leave the series either.