I think that the assymalation did not take place because as someone said b4 the people of Gaia hads to be killed by war.Only that didnt go to plan cos when Garland replaced kuja with Ziddane the fighting slowed down.And when Queen Brahn dies the fighting stops and kuja turns his attention to Ziddane coz he wants revenge.And at the end there is no fighting and the war stops all together.Maybe what we're shone at the end is almost the asymalation of Gaia. I'ts hard to believe that Square would end the game showing the thing you were trying to stop from the begining.