[ sobering? zuko would call that sort of uncertainty terrifying, but he's never been very good at handling that which is up in the air. his own mortality isn't something he needs to contemplate. an existential crisis would do him no favors, especially at a point in his life where he's finally starting to feel at home in his own skin and no longer at odds with himself. ]

The Avatar is reincarnated. One dies, another one is born, and they can communicate with their past lives and draw upon those memories and experiences when needed. They live on, in a way.

[ aang could explain that better. ]