prevent cherry-picking by the forwards, and the game actually becomes more defensive. This has been tested numerous times, and the result is always the same -- more defense, less offense.
What they could do is tweak the rule so that if any part of the offensive player's body is onside (e.g. his back foot is parallel with the last defender), then the player is onside, by definition. Also, I think rebounds off the post or goalkeeper should automatically put all offensive players in onside position for the next touch.