See, that right there is a problem for me. In so many ways your saying that unless that take an american cast - and by american I am going to take a leap and say you mean white american - then the movie has a slim chance of doing well. There is a such thing as Asian-American, do they count too as candidates for the movie roles? Because I get the distinct feeling that when you say american you think only White Americans. There are African-Americans (me), Mexican-Americans (Leonmorado), Japanese-Americans, Chinese-Americans, Vietnamese-Americans... you see the pattern, right? I just don't see why or how choosing to use an Asian cast can hurt the chances of this movie doing well? Especially, since it is a movie based on Asian cultures and themes. Furthermore, a lot of the characters have Asian features, Zuko and Mai definitely look East Asian to me. Katara, Sokka, and Hakoda come of as Native American, Jonathan Nyguen is a close match to Aang and he is Vietnamese. So, I don't understand why someone one would think that the movie will fail because of the choice to use people of Asian ancestry. Is it because you don't think the public will take interest unless there is a variety of ethnicities? Is that it?
As for those movies you mentioned. I get why they choose the people the did for those movies. The Grudge and The rRng were in fact American remakes of Japanese movies, so they chose American actors. This on the other hand is different. This is an American cartoon, based on Asian culture, themes, and appearances. Thus, it would follow that they would stick to the Asian theme and go with actors of Asian ancestry.