I agree with you. Movies don't need a white lead to be successful or to appeal to white people. My favorite movie, Precious, has a black female lead.
The Last Airbender was a trainwreck. The race changes were the worst, but trying to cram an entire season into one movie is well-nigh impossible. The exposition only made it worse. The cartoon was already doing great - they didn't need a movie.
Oh my gosh yes. Aang would've been the only person they could've gotten away with casting as white. But characters like Toph and Zuko were obviously meant to be Northern Chinese. That pissed me off too.