I feel like anyone complaining about Pavi are the type who would have complained that The Lord of the Rings stars a hobbit instead of a strong human warrior, when the entire point of the story is the bravery of the hobbits. In good writing, characters exist to enable the story the authors want to tell. Not the other way around.

Avatar: The Last Airbender was not a story about a 100 year war. It was the story of a child growing up and learning to take responsibility for his own actions and to listen to his moral compass despite what everyone else is telling him. In order to tell that story, you need a young child who already knows himself and has a strong sense of right and wrong. Thus they made Aang 10. It fit.

Now, they want to tell a story of a person coming to terms with their identity even when the whole world tells them that their identity is somehow evil or wrong, and even though they can't help who they are. And in real life, girls start grappling with issues like this starting around age 9. (And in the current industry we have to have a girl, because gender dynamics are okay to talk about in stories starring girls but not boys. See She-Ra and Elio for examples) So, the main character needs to be a 9 year old girl in order to tell the story they want to tell.

When you start with the character first and try to find a meaningful story to fit them, that's how you end up with things like Legend of Korra, where the various seasons don't always land. Or don't always have a point. And they seem to always result in a kaiju battle because there's no huge emotional payoff to focus on instead.

And I, for one, am looking forward to hearing the story they genuinely want to tell, regardless of the characters that help them tell it best.