their rookie contract ends? Isn't that the point of the the rookie scale. Durant led his team to the finals in his 5th season. Of course there are NBA ready talent that have a big impact their first couple seasons (Duncan,Rose,Wade,K-Mart) but their are always GM's who give a max-deal to someone based on potential. (Jefferson,Lopez,Hibbert)
It depends on the player, but generally I would say someone should be entering their prime after their first non-rookie contract. Somewhere between the ages of 25 and 29. I would say that even though Lebron was a great player after his rookie contract he is at his prime right now. I feel it will be the same way with players we think right now are in their prime, like Rose and Durant. Some players do peak earlier, though some of those are because of injury, but I feel overall a players peak should be later than their fourth or fifth year in the NBA. It can also depend on position, it seems to take centers longer to peak, in general, than it does for a sf. Max deals that are given based on potential are more of a product of the salary structure in the NBA rather than a true reflection of a players talent currently. Sometimes you would rather gamble on a player than risk losing him to a rival.
The point of the rookie scale is to (a) keep player salaries low, (b) help lousy teams by giving them good players playing for much less than they are worth, and (c) protect teams from getting stuck with an expensive flop (see Darko, for example). The NFL didn't have a rookie scale for a long time and it got to the point that your top picks were so expensive that they had to basically become All Pros immediately or you were wasteing money.
The league would like to be able to lock players into their origianal team or keep them on a low rookie scale longer, but the players union won't just bend over and take anything. So the rookies scale is currently only three years. After three years you've got to figure that you should have a good sense of how good a player is. But yes, most players hit their prime about 25 or so and that is also when they become an unrestricted free agent or shortly thereafter.
Don't freak out about Max contract money. $20 million a year isn't that much to the current crop of owners. When the new TV deals start kicking in all the teams will be rolling in cash. And the luxury tax? In some ways, who cares. The money paid into the luxury tax just gets divided among the other owners. Some years you might pay the tax, but other years you won't and you will get paid. The luxury tax can't hurt the owners as a group.