Why did the Old Testament law economy slip off the scene?
The Old Testament law economy slipped off the scene because God turned to the Gentiles. An economy centered on Israel under law, temple worship, priesthood, and sacrifices was thereby superseded by something totally different: the righteousness of God without the law now manifested.
Les explains that the Old Testament was predominantly under the economy of law 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. This different revelation is part of what Paul speaks of—the mystery, the secrets held in the mind of God but now being revealed—language not found in the Gospels or the Old Testament.