The players that make it through the filter of playing in the NHL end up making more money because they get paid when they're older, so even if the McDavid's or Crosby's could get $100m at 18, it's that 2nd or 3rd tier of good players who really benefit from that stupid 3rd contract. Even then, if staggered free agency was still how it's done, you're still overpaying for 29 year olds, because a small number of those guys are still going to be the only guys available.
The players simply want owners fighting over a limited pool of them any given year. And the 30 year old looking to get paid isn't going to fight for the 15 year old who isn't even in the union yet.