When mankind was placed on this earth and Adam and Eve fell, God still had a choice of what to do for us. As Adam and Eve had sinned, He could have just cancelled the plan and obliterated the whole situation, according to verse 6. But, as Alma pointed out, it was part of God’s strategy not to do that. He placed instead some boundaries and a flaming sword around the tree of life so they could not lock themselves into an eternally fallen state. He gave laws with blessings and punishments, so that Adam and Eve could make choices.
This point is important for Alma’s logic. Because God chose to bear with us, He did so by appointing that we would die. We would not be stuck in this lone and dreary world forever. Providing us a way to return home to Him was an essential part of His loving plan. And so, He gave us time. We will not know how long our individual times will be, but we do know that we have time, a probationary time. That merciful gift is the heart of God’s great plan of mercy, redemption and happiness.