The few powerful souls who endured the exposure to the elements generally lost their humanity severely, becoming evil spirits, which was a fate worse than death.
However, once you receive the protection of the Underworld, you are almost the same as when you were alive. You can live another lifetime in the Underworld until your soul's strength is exhausted, at which point you'll return to the River of Souls.
If you perform exceptionally well and have a bit of luck, you might even gradually strengthen your soul body, becoming an official in the Underworld. Thus, you would achieve in death what you could not in life, reaching the peak of your ghostly existence.
Moreover, the possibility of resurrection or reincarnation isn't entirely out of reach.
Can those wandering spirits hope for such treatment?
Hence, mastering the Underworld entails immense power, and Dorothy could easily use this power to control all mortals.
