Fun fact: the reason we covet destined death, is that because Destined Drath is sealed in Azula, all life is immortal, people just reincarnate and demi gods are eternal. By consequence so as the Erdtree itself. When we burn it with the forge, the fire does nothing (a spirit of a commoner after Leyendell becomes ashen confirms it) because the Erdtree is immortal. That's why we covet destined death. When we release it finally the Erdtree is no longer immortal and can be burned proper.