This is actually a good idea.
The problem is the players and agents know what kind of money the NFL generates. Merchandise, tickets, DirecTV, PSLs etc have the league and owners simply rolling in the money. Agents push big money. The players want a chunk of all that revenue.
So why do the owners agree to an unrealistic cap? Its obvious they want to have leverage in the player salaries and want to be able to have an argument for "outrageous" pay days. But the truth is, that doesn't stop the teams from signing players to astronomical salaries and contracts, especially when compared to deals just 5-6 years ago.
Its like a double edged sword.