You're looking at a game series, that was so hyped up and important to Nintendo, that they changed plans for it. When Twilight Princess was first announced, it was said it was going to be for the Gamecube. Since, Nintendo knew it was going to be so big and hyped, they made the first release for the Wii. This would help increase/max out Wii sales.
Another thing, to realize, Nintendo takes their sweet time developing Zelda games. They want to keep up a good reputation and don't want to throw out some bad game, just to make a few extra dollars. Zelds is one of Nintendo's longest series, also Zelda is one of the longest series in all of the gaming industry. My final point is, that Zelda has been around a long time, and as long as it keeps making a lot of money, it'll continue to be around.
My only worry is, when Miyamoto retires. Then what will Nintendo do? I hope that doesn't come in my life time, but doubtful since I'm fairly younger then Miyamoto. Hopefully, that won't happen anytime soon.