The point was, in most cases, it's hard to judge what a 21 year old will be throughout his primes. Kopitar, for instance, is getting underpaid. Signing a 7 year at 23, after a down season (for him), the Kings got a bargain PPG'ish player for $6.8M per. Using smaller windows allows more fair deals for both sides. I don't see what the issue of superstars having to "qualify" for new contracts every 5-7 years instead of every 8.
Let's say Sid gets concussed again (*knock on wood*), why should the Pens' owners pay him another $100M for the rest of his non-existent career?
Now, if they were to move off guaranteed contracts to ones based on say base+performance bonuses... it'd be a different story.