When you force the mediocore players to play a new job from level 30, instead of level 50 or 60, you only extend the time it takes for them to reach the level cap. They do not become better players by slogging through another twenty levels. This has been proven and demonstrated time and time again. The players who cause the problem of being mediocore do so intentionally—they do not want to improve, so they do not. However many levels you force them to go through does not change this in the slightest.