Why I think the end of fire is the good ending for the world and humanity:
I will start by going over what happens in the other endings:
Link of the First Flame: The age of the first flame is prolonged even more and eventually leads to Dark Souls 4, the eclipse that looks like the Darksign and the Darksign naturally persists
Usurpation of Fire: The Unkindled absorbs the flame and proceeds to rule as a Hollow King using both Dark and Fire as power. The eclipse that looks like the Darksign persists and safe to assume so does the Darksign as that's the power source of the Hollows in DS3. The first flame is still alive and would likely get to the point of another fading once the Unkindled eventually dies/gets overthrown.
The Unkindled Ending(TEOF Alternate): The Unkindled absorbs the flame to himself. The eclipse that looks like the Darksign this is the same as the UOF ending so it's the safe to assume that the Darksign itself also still persists as there was no extra or different actions taken by the Unkindled that will negate it. The first flame is still alive and would likely get to the point of another fading once the Unkindled eventually dies.
What I learn from this is that the eclipse that looks like the Darksign is symbolism for the Darksign itself to indicate to us that the Flame still holds back the Dark and Gwyn's curse wasn't broken, so the Undead remain Undead and eventually Hollow. But how would Gwyn even have the power to hold the unlimited Dark Soul even eons after his death? He used the first flame to do so, that's why the dark sign looks like a ring lit on fire and why it forces the Undead to be drawn to bonfires in order to keep their humanity. It is operated on a way similar to The One Ring from LOTR; the Darksign forces the undead to seek and need fire or else they would risk going Hollow, which is the defense mechanism bestowed by Gwyn so the Undead wouldn't want to let the flame fade. But what if letting the flame fade is the cure? The First Flame never faded before TEOF ending and the first flame remained powering the Darksign, so there was no way for the undead to even know that letting the First Flame fade is the answer.
On The End of Fire ending: The First Flame fades so does every source of fire in the world which includes the Eclipse that symbolizes the Darksign and the Darksign itself, which was powered by the First Flame. The undead no longer held back by the Dark Sign can finally find their true form and evolve. The world reaching the Age of Dark is no longer stuck in the start of a cycle and balance returns with the second Age of Fire long away in the horizon in the form of tiny flames dancing on the darkness. The world gains his natural order and the Undead are finally no longer oppressed by Gwyn.
