Had Adam partaken of the tree of life after his fall, he would have had instant immortality. On the surface, this sounds great! But he would have been immortal in a fallen state. He would have condemned himself to eternity in a telestial existence with its spontaneous thorns and thistles and food supply contingent upon the sweat of the brow (Moses 4:23-25). This was not according to the Lord’s plan and could not be allowed.
Therefore, we learn that death is a blessing. Clearly, the mythical search for a fountain of youth is a misguided adventure. This mortal probation with its pains, travails, and mistakes can only end with the blessing of death, whose sting was swallowed up in Christ’s infinite atonement. All this is according to the great plan of salvation which God would not allow to be frustrated.
Bruce R. McConkie
“Alma said, ’the word of God would have been void, and the great plan of salvation would have been frustrated.’ Adam must fall; Adam must die spiritually; Adam must repent and keep the commandments and live again spiritually; Adam must die temporally; Adam must be raised in the resurrection -- all this must be or he could not be saved.” (A New Witness for the Articles of Faith, p. 88)
Bruce R. McConkie
"Such is the divine will. Fall thou must, O mighty Michael. Fall? Yes, plunge down from thy immortal state of peace, perfection, and glory to a lower existence… Yes, Adam, fall; fall for thine own good; fall for the good of all mankind; fall that man may be; bring death into the world; do that which will cause an atonement to be made, with all the infinite and eternal blessings which flow therefrom.
“And so Adam fell as fall he must. But he fell by breaking a lesser law-so that he too, having thereby transgressed, would become subject to sin and need a Redeemer and be privileged to work out his own salvation, even as would be the case with all those upon whom the effects of his fall would come.” (The Promised Messiah, pp. 220-21)