Never, because the basic trope of "Avatar" is as old as there have been story tellers gathered around campfires even before writing was invented. The characters in "Avatar" were only this incarnation, but the story will continue to please people as long as someone is oppressed, or thinks he is, and is in the way from whatever the powerful antagonist wants. Doesn't matter if it is Unoptanium, gold, oil, land, the captains chair, access to females, or the number three AP ranking, the story is basically same. James Cameron told it in a very good way, and deserves his success.
January 4th, Go Horned Frogs