Kuyper’s 16^th chapter concerns two related concepts: First, he considers the duration of human life and how that has changed since the Fall. Second, he considers the question, What would have happened if Adam had not sinned.

As to the first, he notes that human life has fallen off significantly in duration and vigor since the time of the Patriarchs. A matter unknown to Kuyper is that recent genetics research has demonstrated that human life has in fact degenerated.

The work of John Sanford on this point has been remarkable. He has coined the term “genetic entropy”. In short, as genetic material is duplicated (which is necessary for both our own continued existence and for the continuation of the species) it accumulates errors:

“What is Genetic Entropy?  It is the genetic degeneration of living things.  Genetic entropy is the systematic breakdown of the internal biological information systems that make life alive.  Genetic entropy results from genetic mutations, which are typographical errors in the programming of life (life’s instruction manuals). Mutations systematically erode the information that encodes life’s many essential functions.” 

And thus as we have continued on, rather than becoming stronger, we become weaker as individuals and as a species. That our life expectancy has decayed is simply a matter to be expected.

Lest one think that this is merely cherry picked creationist reading of the evidence I proffer, “That Classic Image Everyone Uses to Illustrate Evolution Is Actually Wrong” published on sciencealert.com, March 8, 2020:

Yet this is one of the most predominant and frustrating misconceptions about evolution. Many successful branches of the tree of life have stayed simple, such as bacteria, or have reduced their complexity, such as parasites. And they are doing very well.

In a recent study published in Nature Ecology and Evolution, we compared the complete genomes of over 100 organisms (mostly animals), to study how the animal kingdom has evolved at the genetic level.

Our results show that the origins of major groups of animals, such as the one comprising humans, are linked not to the addition of new genes but to massive gene losses.

This loss of information results in a degradation of human life. This degradation was not the original plan, but came in as the result of sin. Rom. 5:12.

What then would have happened had Adam not sinned? Perhaps he would not have died, but was the Garden simply the beginning and the end of the story of humans. Kuyper argues for a progress based upon a comparison of the Garden and what God has prepared for humanity in its culmination.

He notes the mutability of humanity and the possibility of sin which exists at the time of creation and Adam’s existence in the Garden. This is apparent from the fact of a test and a fall. This contrasted with the Kingdom to come, where “not only is there no sin, but any entering of sin is utterly inconceivable.“

Along this same axis of comparison, he notes that Paradise could be lost, but the eternal Kingdom will not fail; human nature could be corrupted in the Garden, but it will be established upon a sure foundation in the kingdom to come.

From this we can conclude that Garden, though very good was not the permanent condition.