What happens if Avatar: Fire and Ash doesn’t make enough profit to convince Disney to bankroll Avatar 4 and 5? Creator James Cameron has said he’s ready to walk away from the franchise, adding he’ll write a book to resolve the one thread Avatar 3 leaves open.
I would say one of the key points is that Aang gets constant support from everyone around him. Like any individual, he’s nearly powerless without that support. Most other “chosen one” stories I’ve seen, the character is saving everyone else.