1. Ranking is base on how fast you progress instead of how far have you progressed.
2. A few artificial players to snap all the good airports before any real players in the beginning of the game. The AI players can slowly fade out once competition gets intense.
The benefits are:
1. We don't have to worry if there is no world restarting for a long time, because there is always a good world you can join.
2. The world can last for years, you don't have to play the entire lifetime.
3. Everybody can play his favorite era, we don't have to create so many different worlds just because everyone likes different era.