The cultural aspects of the cartoon was to give the characters a basis of origin for their bending styles, it was never the main focus of the show. Due to that I'm sure many viewers didn't ever know the cultural difference of the characters.
The point of the show transends any one culture. The point was these children backed the Avatar because they all believed in uniting all the people of all the nations regardless of their culture, heritage or race. If the massage is bringing all types of people together in harmony, then it shouldn't matter what race or culture the cast is because their ideals represent all of us.
I see what you're saying, and it'd be fair enough if we could make an adaptation of Avatar in a cultural vacuum.
Unfortunately, we can't. Whitewashing characters or 'normalising' them to better suit the demographic Hollywood imagines will fail to respond to a character of colour is an ongoing problem. So to my mind, a message of bringing all nations together regardless of culture is tainted when it's told by once again rendering invisible faces which aren't part of the one culture which is rampantly dominant in popular entertainment, to the exclusion of diversity.
Colourblindness is too often used to mean 'pretend everyone's white'.