The Old Covenant is "eternal" in the sense that Christ fulfilled it — fulfillment, not destruction, transitions to the New Covenant

Jesus said he came not to destroy the law but to fulfill it (Matthew 5:17). On the cross, Jesus perfectly obeyed and completed the Old Covenant. Those who are identified with Christ are therefore reckoned as having fulfilled it too. The covenants are not in conflict — the New Covenant is prophesied within the Old (Jeremiah 31). Mike uses Paul's analogy in Galatians: the law as a schoolmaster (paidagogos) that leads to Christ. Once maturity in Christ is reached, you no longer need the schoolmaster's authority — though you keep the lessons.