Doesn't TF2PlayerRankings award ELO or whatever it is based on the skill level of the people you're playing against?
How are newer players going to get into the top 20% if they can't play against better players because those ones are already playing advanced lobbies? Plus that 20% would be a sliding scale wouldn't it? And people who make it to the top 20% and play advanced lobbies will only further cement their position there.