For three thousand years, war after war took place. That wasn't to say there weren't periods of peace, but they were rarely long enough to be significant.
By the time the Neurics gained their advantage, their population had once again ballooned to over ten million.
And, when they took the advantage, they didn't let it go.
Rather than defending as they had been for the past two thousand years, the Neurics went on the offensive. They invaded other continents, influencing their people and seizing control.
They waged war on everyone and everything, and after losing and suffering for thousands of years, they had learned how to win.
Slowly but surely, they took control over every continent in their vicinity.
They subjugated almost all of the cultivators. After all, not many were actually loyal to their sects. Most were only in those influences to further their own cultivation, and in that sense, the Ignis Empire was simply superior.