Well, right now there already is pretty profound overcrowding in some leagues. Take Gold for example - you can go 0-3 in Bronze, Silver, or Platinum and you'll get one more game, if not two. (I've seen Plat decks appear before me at 0-4, for certain. I'm not positive about Silver, but I don't remember having problems there.)
You go 0-3 in Gold and you're done. Heck, 0-2 puts you on the brink.
So the overcrowding is already there. I think that the rewards for winning at the higher leagues need to be increased if we're to encourage people not to go to the league with the most play (highly likely to be bronze) rather than the best they can reach, but even if we don't do that, I'm guessing some people in Gold would step back to Silver if only to get out from under the brutal competition at that league level.