Many people believe that God meant life in this world to last forever, and that physical death was appointed by God because mankind sinned. Some even believe that eternal life will be unending life in a restored garden of Eden on this planet.
Personally I doubt that man was ever meant to live on earth forever, but we'll not worry about that now. However you might be interested to think about the following...
Firstly consider this passage: "Through one man sin entered the world, and death through sin. And so death spread to all men because all sinned" (Romans 5:12).
Now we need to ask what kind of death resulted from sin —was it physical death, spiritual death, or perhaps both? You might well say to me, "Physical death I know about, but is there any such thing as spiritual death?" Well Paul thought so.
Paul says, "For sin, taking opportunity through the commandment, deceived me, and through it killed me" (Romans 7:11).
Paul says to the Ephesians, "You were dead in your trespasses and sins" (Ephesians 2:1). Paul and the Ephesians were not physically dead. Yet they were dead. So they must have been spiritually dead.
This would explain a problem in something God said to Adam when he commanded him not to eat of a certain tree in the garden. God said, "In the day that you eat from it, you shall surely die" (Genesis 2:16-17).
Adam disobeyed God, and ate of the tree. But he did not die that day, that is to say he did not die physically in the day that he sinned. In that day, he died a spiritual death.
And I think this is much more than a metaphor for losing his innocence. Adam became spiritually dead in a very real and literal sense. He became "alienated from the life of God" (Ephesians 4:18).
We see therefore that Adam's sin impacted on the state of his soul much more than on the state of his body. We must seek another reason for the death of his body hundreds of years later. And that reason is clearly stated in what God said to Adam...
God said to Adam, "By the sweat of your face you shall eat bread till you return to the ground, because from it you were taken. For you are dust and to dust you shall return" (Genesis 3:19).
Notice that God did not say, "You shall return to dust because you sinned." Rather, he said, "You shall return to dust because from it you were taken". So it seems that the physical body of man was created from dust, and therefore it goes back to dust.
Physical death is, primarily, due to the manner in which God created man, not to the fact that man sinned against God.
This leads us to understand that the tree of life (Genesis 2:9, 3:22-24) was not for the life of the physical body. The trees God planted for food gave life to the body.
The tree of life imparted the gift of eternal life to the innocent. It did not impart forgiveness to sinners. Therefore only the innocent were entitled to eat of it. Consequently, Adam and Eve were banished and barred from the tree of life.
Of course, we know that God had already planned for a means by which sinners could overcome spiritual death and regain eternal life by being born again. To make this possible, God had to send his only begotten Son into the world to suffer physical death as a sacrifice for sins (John 3:16, Ephesians 1:3-7, 1Peter 1:23-25).
We have seen that the death sin causes is spiritual, not physical. We have seen that physical death is caused by having been created from dust. Does this mean that if man had not fallen he still would have died physically and gone back to dust? Let's consider this...
"By faith Enoch was taken up so that he would not see death, and he was not found because God took him up. For he obtained the testimony that before he was taken up he was pleasing to God" (Hebrews 11:5).
Enoch was only half way through life when he was taken up to heaven. God cut short his earthly life so that he did not see old age and death. God took him into eternal life in heaven.
I cannot prove it to you absolutely, but it seems sensible to me that God would have done for all mankind what he did for Enoch. "Enoch walked with God, and he was not, for God took him" (Genesis 5:23-24).
Perhaps this journey and blessing would have been granted to Adam and Eve and to all their offspring, if only they had remained pleasing to God. But the fall of man made this blessed journey an extreme exception instead of the normal course of man.
There is no indication that this world, even in its pristine and unspoiled form before the fall, was meant to be the eternal dwelling place of human beings. They were always meant to spend a finite lifetime on earth, and then spend eternal life in heaven with God.
So even though we found that sin causes spiritual death, and being made of dust is the reason for physical death, it nevertheless seems true that physical death is a curse upon man as a consequence of sin and the fall, because man lost the blessing of being taken up into heaven so as not to see death.
Fortunately, even though we may suffer physical death and walk its shadowed vale, we nevertheless have the hope of resurrection from the dead and of being taken up into heaven when Jesus comes (1Corinthians 15:26,50-57, 1Thessalonians 4:13-18).