It's our basic morality - ability to die, a material consequence of a spiritual reality: dead in sin, eventually physically die as souls, too. Average lifespan since Eden has steadily dropped, according to lowered life expectancy - God setting the decline precisely. Mortal death is natural for mortals, not so?! - everlasting annihilation is extra.

Teresaq, if I may share a thought. The reason sinners die a slow, gradual first death is because God denies them access to eat of the tree of life. Otherwise, sinners would live forever. Which is not to say they cannot be murdered.

"And the Lord God said, Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live forever: therefore the Lord God sent him forth from the garden of Eden, to till the ground from whence he was taken. So He drove out the man; and He placed at the east of the garden of Eden cherubims, and a flaming sword which turned every way, to keep the way of the tree of life." (Genesis 3:20)

Thus, the Bible teaches that sin does not kill sinners. Sin is not an active, independent agent that goes around killing sinners. Yes, the results of certain forms of sinning can lead to death. But this isn't true of all sins. For example, if two people love each other and decide to live together without getting married it will not result in death. That is, living in sin in this way will not kill them.

so, was it "natural mortality" that made mens longevity shrink so dramatically so quickly?

After the flood the people ate largely of animal food. God saw that the ways of man were corrupt, and that he was disposed to exalt himself proudly against his Creator and to follow the inclinations of his own heart. And He permitted that long-lived race to eat animal food to shorten their sinful lives. Soon after the flood the race began to rapidly decrease in size, and in length of years. {CD 373.3}

After the fall, the eating of flesh was suffered, in order to shorten the period of the existence of the long-lived race. It was allowed because of the hardness of the hearts of men. {TSDF 68.3}

Teresaq, ..., it is the results of sinning that can lead to suffering and death. ...

In Ps. 119:172 the Psalmist thus addresses the Lord, "My tongue shall speak of Thy word, for all Thy commandments are righteousness." The commandments are righteousness, not simply in the abstract, but they are the righteousness of God. For proof read the following:

Lift up your eyes to the heavens and look upon the earth beneath, for the heavens shall vanish away like smoke and the earth shall wax old like a garment, and they that dwell therein shall die in like manner; but my salvation shall be forever and my righteousness shall not be abolished. Hearken unto me, ye that know righteousness, the people in whose heart is my law; fear ye not the reproach of men, neither be ye afraid of their revilings." Isa. 51:6,7.

What do we learn from this? That they who know the righteousness of God are those in whose heart is His law, and therefore that the law of God is the righteousness of God.

This may be proved again, as follows: "All unrighteousness is sin." 1 John 5:17. "Whosoever committeth sin transgresseth also the law; for sin is the transgression of the law." 1 John 3:4. Sin is the transgression of the law, and it is also unrighteousness; therefore sin and unrighteousness are identical. But if unrighteousness is transgression of the law, righteousness must be obedience to the law. Or, to put the proposition into mathematical form:

Unrighteousness = sin. 1 John 5:17. Transgression of the law = sin. 1 John 3:4.

Therefore, according to the axiom that two things that are equal to the same thing are equal to each other, we have: Unrighteousness = transgression of the law

...which is a negative equation. The same thing, stated in positive terms, would be: Righteousness = obedience to the law. (Christ and His Righteousness)

hopefully meaning justification by faith is both legal and the more so experiential

If J&S are jointly meant by this "righteousness", then it's nothing new and they side-stepped this gem of Gospel power truth despite coming so close to stating it.

Does hinge on Christ taking sinful humanity for us to have this experience of justification by faith.