While valid points, here are facts about me. I don't care whether the whole cast loses or the Phoenix Kingdom reigns supreme. It is my staunch belief that I, as GM, create this world and am here to tell a story as it unfolds in it. Whether that story is of uplifting success or tragic defeat is of no consequence to me.
Characters should be able to advance without needing Advanced Bending Styles. As a matter of fact, some literally never develop them. Regardless of progression or attempts, some characters might just be inept in terms of learning to Lavabend, plantbend, metalbend, sandbend, bloodbend, etc.
Not all characters are created equal. It is my stance that a character is meant to be an influence on others and the roleplay, not become powerful. Influence may mean influencing those whom are powerful. It may mean using a unique skill when the time comes. It may mean forfeiting one's life when the situation calls for it. This is as much a literary fabrication as it is a whole new world.