Ibba Qian was not the only Fallen.
The waters of the Flowing Gold River are the manifestation of vast time. They are not an attack, not corrosion, but erosion. Even if your ideals are as strong as rocks, they will be worn away by dripping water; even if your realm is prosperous and powerful, it will become the debris of history.
It is impossible for humans to catch up with time or to resist it.
Ibba Qian was just an outpost; after him, in succession, legends became stagnant in the Flowing Gold River. No matter how hard they tried, they were eventually washed away. They might have had many great ideals, such as continuing to follow the Great Skill of the Gray Fox Era, such as pushing the boundaries of magic, such as probing the forbidden zones of angels, such as... But in the Flowing Gold River, their ideals were eroded until nothing but a blank space remained, and then persistence was no more, and stagnation ensued.