When Jesus comes, it is more than a book end on a failed age: 4000 years where God used various strategies to get man, especially Israel, to do what he had planned all along. If that were the case, I believe God could have had Jesus go to the cross almost immediately in history. So I view the preceding history, as necessary, not accidental, even if we don’t fully understand all of the reasons for it.
When Jesus comes it is a direct confrontation of Satan’s power. It is also the highest stakes moment in history. God has not only submitted himself to a human form, but also to human weakness. It was at least theoretically possible for Jesus to succumb to Satan’s temptations. And if he had, not only would evil have triumphed, I believe the very fabric of the universe would have been destroyed as the Godhead would have been split. Just the opposite happened, however. Jesus resists every temptation of the wilderness, and this gives him authority over the devil. This authority does not extend beyond Himself and those under his direct authority, however. A final victory can only be won by actually setting the entire human race free from slavery to the devil. Jesus reverses the fall by actually proactively choosing good at every point that Adam and Eve chose evil. This releases God of the obligation to damn everyone because of their sin. Satan no longer had right of ownership over them, but anyone who would excercise faith in Jesus effectively reverses the effect of the fall on their own life, and destroys the devil’s authority to rule over them and drag the to hell in this life and the one beyond.
Furthermore, this has a corporate effect — as people are able to walk in direct fellowship with God and express his authority wherever the go. Now instead of just God spreading truth or maintaining a strand of his own people, an army is unleashed. Man has his original authority over the devil back. Wherever he goes, he will push back the Kingdom of darkness. This is first and foremost by depriving it of citizens and bringing them in to the Kingdom of light. It has secondary effects, however which are seen in the way the world itself operates. Each historical advance of God’s Kingdom creates a change in geopolitical systems.