If US consumers have 1 billion dollars to spend on sports/media/entertainment and the NFL is getting 65% of that and they want to get richer they need to either take a % from another sport/artist/entertainment or go out and find a new dollar to take.
They can spend $.75 to convince someone who is not spending on the NFL (either because they're not spending on entertainment or because they're spending on OTHER entertainment) to spend a $1. Here they make a profit of $.25. OR they can go to Europe where it only costs $.35 to get someone to either spend a saved dollar or to divert a dollar spent elsewhere (soccer perhaps) on the NFL thereby making $.65.
The NFL has soooo much money that they're probably doing both and will until the margin is spend $.99 to make a dollar.