Why must God replace the old Adam with a new nature?
At salvation, the old Adam is crucified, and something must immediately replace him; otherwise the believer would be an empty entity. God therefore replaces old Adam with the new divine nature, a creative work from God, so the believer is not left empty but possesses God's divine nature.
Les explains that chapter 7 is Paul dealing again with the same issue: how to deal with old Adam over against the new divine nature Christ places in us at salvation. When Adam is crucified, something had to immediately replace him, or the believer would be an empty entity. God therefore replaces old Adam with the new nature—the divine nature that comes from God by a creative work. Les also notes that law has dominion over a man as long as he lives. This is not only the Mosaic law but law as it governs society, like constitutional law over U.S. citizens. At death, that government loses all control; only death severs some things, which is what Paul is driving at.