The problem in Eden was that with agency and the imposition of opposition in all things, there was not only the possibility, but the probability, of sin. Had Adam and Eve eaten of the tree of life they would have lived forever. Without death, there was no boundary between mortality and immortality, and therefore no separation between the two in order that there would be a restoration. No restoration could be done, because nothing was lost that could be restored. Without dying, there is no resurrection -- that is to say, with no death of the sinful life, there is no resurrection to new possibilities.
The blessing of prohibiting the partaking of the fruit of the tree of life was to allow for repentance. When Alma says that “there was a time granted unto man to repent,” he refers to our mortal lives. In that time, we can learn the important lessons of mortality. Even when we sin, we can learn repentance. Then, because death ends that state, the doctrine of restoration allows what we have learned to rise with us and, therefore, to lead us to an eternal life that is eternal in quality and not simply lengthy in time.
The conclusion of this discussion of the benefits of mortal death is that agency has time to work: “thus we see they became subjects to follow after their own will.”