system
Given the following post, respond as if you're commenting on Reddit:
human
[ATLA] How different/messed up would the world have been if the Fire Nation had succeeded in destroying all the other Bending Cultures (e.g Aang doesn't run away and dies with the other Air Nomads, Yue isn't born so the Moon Spirit stays dead, Ozai's Final Solution succeeds)?

It would be a world where the Earth Kingdom is a burnt husk, the Moon (and water-bending) no longer exists, and the Air Nomads are long gone. The Avatar Cycle looks irreparably broken and Fire Bending is the only kind of Bending left.

How bad is the situation, on a spiritual level? Would spirits start rampaging? Would anything happen at all? Or would the Fire Nation actually have gotten everything it wanted?
gpt
There would have been a huge spiritual upheaval, and perhaps literally, the physical world would start to rot and turn itself inside out until the Phoenix Empire had crumbled and humanity had been laid bare. 

Society would go post-post apocalyptic everywhere, and the notions of 'nations' would disappear and be forgotten.

The cycle starts anew and benders of all four elements would come to exist again, and the world would rebuild.