The trade off here is that draftees get more money than they have "proven" they deserve, in exchange for giving up the right to go to whatever team they want by going through a draft process determined by the league that gives them almost zero control of their fate. The propspects would certainly have an interest in finding out who would be the highest bidder, where they could be a starter, what region they would live in, how cash strapped the franchise is, whether the team is a contender, etc. In order to get them to give up all those considerations, the league guarantees that a certain amount of money will be spent on rookies and that they will be paid based on their "potential" as determined through their draft position.
As for contracts being adjusted based on performance, there are all sorts of likely to be earned incentives and unlikely to be earned incentives that teams use in contracts to encourage/reward performance. They are generally more attuned to individual performance such as Henson's deal that has contingent clauses if he takes 30% or 60% of the snaps in the first 3 years of his deal. Playoffs or any team record would be a poor contract basis as it is a team sport and it just isn't realistic or fair to think the QB alone much less the strong safety can elevate their individual performance sufficiently to determine the difference between a losing season and the play-offs.
The Art of War