why not destroy satan?

John Piper in Spectacular Sins and Their Global Purpose in the Glory of Christ on why didn't God simply destroy Satan at the beginning?

The ultimate answer . . . is that “all things were created through Christ and for Christ” (Colossians 1:16). God foresaw all that Satan would do if he created Satan and permitted him to rebel. In choosing to create him, he was choosing to fold all of that evil into his purpose for creation.

That purpose for creation was the glory of his Son. All things, including Satan and all his followers, were created with this in view. They were created knowing what they would do, and that knowledge was taken into account in God’s decision to create them. Therefore, the evil that they do in the world is part of how the greatest purpose of God will be accomplished.

Satan’s fall and ongoing existence is for the glory of Christ. The Son of God, Jesus Christ, will be more highly honored and more deeply appreciated and loved in the end because he defeats Satan not the moment after he fell, but through millennia of longsuffering, patience, humility, servanthood, suffering, and eventually his own death. A single, sudden, and infinitely holy display of power to destroy Satan immediately after his fall would have been a glorious display of power and righteousness. But it would not have been the fullest possible display of all the glories in the Son and the Father. God chose an infinitely wise way of displaying the full array of divine glories in letting Satan fall and do his work for millennia.

The glory of Christ reaches its highest point in the obedient sacrifice of the cross where Jesus triumphed over the devil (Colossians 2:15). Jesus said in that final hour of his own sacrifice, “Now is the Son of Man glorified, and God is glorified in him” (John 13:31). Paul said that the crucifixion of Christ is the point where we see his wisdom and power most gloriously displayed: “We preach Christ crucified . . . the power of God and the wisdom of God” (1 Corinthians 1:23–24).

Jesus said to Paul about Satan’s thorn in Paul’s side, “My power is made perfect in weakness” (2 Corinthians 12:9). Satan, and all his pain, serves in the end to magnify the power and wisdom and love and grace and mercy and patience and wrath of Jesus Christ. We would not know Christ in the fullness of his glory if he had not defeated Satan in the way he did.