The prolonging of the time on the earth is an essential part of the plan. It was never in God's plan that Adam and Eve would die immediately upon eating the fruit, and even in this Satan told no lie. The prolonging of life becomes essential so that there is time to have both the benefits of the ability to act upon opposites, but also to overcome the separation from God that also accompanied the Fall. Lehi states that Adam and Eve are given commandments, but specifically mentions only the command to repent. In the logic of Lehi's discourse, this is the essential commandment, for it is the one that allows for the reconciliation between man and God. With the command to repent, Lehi now closes the circle on the theme of Christ's mission (verses 6-10). He began with the essentiality of Christ's mission, and now has provided the background to demonstrate that necessity.
With this, the development of the discourse is completed, and Lehi now puts on the finishing touches.