FAIR WARNING: These are my thoughts after beating TRC and letting it sit for a few days. I'm not an expert on the lore, but I love this series and still wanted to share my thoughts on it despite the contradictions there may be. I also don't know if this stuff was already obvious to everyone else or said before, so I apologize if that's the case.
Gael felt like the final boss of the series long quest for the dark soul, everything about the fight feels epic. And at the real end of the quest, you deliver the paint needed for a new world. I want to argue that the conclusion to this quest was only the climax of the story, however, not the ending to the series like I've heard some suggest.
I want to argue that the true ending was the one where you kindle the flame and sit at the last bonfire, waiting for the last embers of your world to fade and die.
Soul of Cinder was the true last boss, not Gael. It held the pride and despair of the lords and Age of Fire, with a theme song whose tone progressively shifted to represent the weariness that has enveloped the world. Even if you aren't the last Lord of Cinder, there wouldn't be much of a point to the next one. Everything worth fighting for is on the brink of fading away. Better to bring an end to the age, the Ashen One must think.
But you meet the painter and hear of her desire for a new, wondrous painted world. How could you not believe in her? Look at where you are. Thus, you begin the quest to finish the painting. Any past quests you had–to become the Lord of Hollows or extinguish the flame–are now abandoned. They are simply nothing compared to what the painter suggests.
After all, your new quest is to ensure the creation of a new world–one untouched by the cowardice of gods and the greed of men.
You find Father Ariandel to show the painter flames in the cold and dreary lands of the painted world. Surely the flames of he who wishes to save a world must be the brightest.
With light comes dark, however, and the most suitable darkness to accompany the dazzling flames of Ariandel and Friede would surely come from legend. You follow Gael's footsteps and begin seeking the Dark Soul. This takes you many years into the future, where the world has become twisted and grey with ash. At the end of it all, you meet an old and true friend, finish the last of a species, and finally see what Gael's quest has made him become.
In the end, he succumbed to the greed of men, his quest having no hope of being completed. The task falls completely to you. However, even when you return to the painter, you don't tell her of Gael's fate. He is to be remembered as the loyal knight he was, no matter how lowly his status may have been.
Once you complete the quest by delivering the Blood of the Dark Soul, you go to the Kiln of the First Flame. You've seen the future, you know that the world still survives, and you know what your decision must be. You must kindle the flame, not to prolong the Age of Fire, but to provide enough time for a new world to be painted and born.
Your duty is to watch the world die and ensure that this weary age persists. This, you must do, else the future in which your past self takes the Dark Soul will never happen.
The story of Lordran, Drangleic, Lothric, and all things past are finished. The quest for the Dark Soul was a long one, but rather than being the end those gods feared it would be, it became the beginning of a cold, dark, and very gentle place.
And as we all know, ash seeketh embers. While the light of a new world was too bright for the Ashen One, the embers of their dying world were just enough to kindle the next.
This ended up getting pretty long, but I hope that it wasn't too boring a read. Thank you to all who got through the whole thing.