Check out this excerpt from Seeing And Savoring Jesus Christ by John Piper (download the whole book as a .pdf file for free here).

The title of the chapter is “The Glory of Rescuing Sinners, Not Removing Satan“:

Now without sin and law to condemn and accuse and oppress us, Satan is a defeated foe. He is disarmed. Christ has triumphed over him, not by putting him out of existence, but by letting him live and watch while millions of saints find forgiveness for their sins and turn their back on Satan because of the greater glory of the grace of Christ.

It was a costly triumph. But God’s values are not so easily reckoned. If God had simply terminated Satan, then it would not have been so clear that God is both stronger and infinitely more to be desired than Satan. God wills for His glory to shine forth not only through acts of physical power, but also through acts of moral and spiritual power that display the beauty of His grace with lavish colors. To take sinners out of Satan’s hands by virtue of Christ’s sin-bearing sacrifice and his law-fulfilling obedience to the Father was a more glorious victory than mere annihilation of the enemy.