Satan does not have complete control now. The Bible is clear that only God is sovereign. Satan has certain abilities and his control is limited by God. Read the first two chapters of the book of Job and it is clear that Satan could only do what God permitted. Currently, Satan has certain powers. He is described as “the prince of the power of the air” (Eph 2:2). He and the fallen angels who follow him are called the devil, rulers, cosmic powers, spiritual forces of evil (Eph 6:11-12). There is no question that he is extremely powerful as described in Matthew 4 when he tempted Jesus. This does not mean he is in control but is allowed to rule in a limited fashion.
What is more important is to understand that Jesus’ death on the cross dealt a serious blow to Satan’s power. You see, fear of death is something that either haunts or should haunt anyone who is not a Christian. That fear is prompted by the fact that our sins are a debt that must be paid. Most people instinctively know they are not perfect and will have to pay for their sins in eternity. But Jesus’ death on the cross wiped out the debt for those who turn to him in faith, repent of their sins and live for him instead of themselves. This act of Jesus disarms Satan and his demons because Jesus defeated death and has triumphed over Satan (Col 2:14-15).
Yes, Satan will be given much more leeway for seven years. That will occur in the future when God pulls back his restrictions on Satan. However, that is not the end of the game. Jesus will put him to death when he comes back. (2 Thess 2:3-12). Remember that God is sovereign. Even while Satan thinks he is in total control for seven years, God is the one who direct all things. The end will come just as he has said.