Alma lays out the basic plan of salvation. The first important point is that the plan of salvation was in effect before the foundation of the world. It preceded the Garden of Eden, and the events of the Garden are built upon the foundation of that promise of salvation, not the cause for its later implementation.
If Adam and Eve, and all humankind, were to live forever in the mortal state, there would be no division in time, no chance at a change of status. Thus, they would not have the opportunity of entering into God’s presence, for they would be forever linked to the temporal state. Death is an essential part of the plan of salvation.
This life is not meant to be permanent; it is meant to be a temporary training ground for something different, something indescribably better.