Why did the law economy slip off the scene?
The law economy slipped off the scene because God turned to the Gentiles, causing that Israel-centered system of temple worship, priesthood, and sacrifices to give way to something totally different: the righteousness of God without the law, now manifested and revealed.
Les teaches that the Old Testament was predominantly under the economy of law given to the nation of Israel, who were under the law with their temple worship, priesthood, and sacrifices. But now that God has turned to the Gentile, naturally that economy had to slip off the scene, and we have something totally different. The righteousness of God without the law is manifested, being witnessed by the law and the prophets. Les emphasizes Pauline teaching for today, but never says to forget the rest of Scripture; the Old and New dovetail together. This is part of the revelation Paul speaks of—the mystery, secrets held in the mind of God, now revealed. You won't find this same language in the Gospels or the Old Testament.