This untenable situation of a mankind inevitably facing both a temporal and spiritual death was tolerable as long as there was a way out. What Alma now does is to show that it is Antionah who has misread the scriptural text. Alma has first shown that the condition of mankind after the Fall was untenable.
Now he shows that the statement of the Lord about the reason for the flaming sword was specifically to create the ability to have a "way out" of the problem. Had Adam and Eve been able to eat of the fruit, they would have lived forever.
There would have been no change of states, so the fallen body would be theirs forever, not the resurrected body. While it would live forever, it would not be changed. If we were to be allowed to go through this life without changing, it would defeat the entire plan of God. In Alma's terms, it would so destroy God's plan, that it would have had no effect upon us at all.