But there is education in a league system.
At each step, newbies come in, learn by playing with strong opponents. They become better, begin to win, the newer players in the step learn from them. Once the player level is really above the current level, he climbs a level. It's like in the educative system, you do have different classes with different levels. The league system is exactly the same.
Moreove, in the current ladder system, a smart guy can check people in the games before joining, leaving the game if it finds people better than him, in order to have greater chances to win the game. In a league system, the gap between the levels of different players is less strong so it discourages his bad behaviour.