Yes, Avatar is critized for just retreading the Pocahontas storyline. But it’s a story after all, a bit of narratization is needed. As for the fact that they need the ‘big strong human to protect them’, yes, that is woven into the story too. Jake has both insider knowledge and intel. He not only knows what the human military contractor is going to do, he also knows the motivations behind their action, how far they are willing to go and what it will take to stop them. As such, his input is crucial to rally the tribes and get nature on his side. Only with the help of Grace’s memories is he able to rally Eywa to their side.