"That might be true, but that might not be true, too. You never know when a brilliant idea will hit." Maybe Aang hadn't been jaded by the island enough yet, but he couldn't imagine not finding a way out.
"That is an interesting thought. I doubt we'd get replaced with copies. Someone would see through that fairly quickly. Though for now, we are here, so until we can find a way home, it seems like what we can do is try to make the best of it. Besides, my friend The Mechanist does lots of experiments, and eventually all experiments end. Then we'd go back, or our spirits go back, or however we ended up here goes back to where we belong." Though Aang decided at that moment not to think of the experiments that ended in large, lab-destroying explosions. That image was not helpful at that particular moment.