How thin would the field be with that rule? You just threw away a decent portion of the class. This year was a bit of a special case with a handful of guys making an early leap to a significant role, but it's not always going to be like that. With a cutoff of 21, you're punishing what has always been the classic route of NHL development, spending a handful of years in the minors learning the game. If you don't want to call those older guys rookies, you may as well throw away their restriction for entry level contracts. How's that for one hell of a can of worms.