The question was asked, “What if Satan defeats Michael?” The answer is that Satan can never defeat the Archangel Michael, as he did not defeat Michael the first time around, because the Michael has the full power of God behind him, whereas Satan does not. Saint Michael is the prince of the heavenly hosts while Satan is the devil. Furthermore, Scripture states:
Then I saw an angel come down from heaven with the key of the Abyss in his hand and an enormous chain. He overpowered the dragon, that primeval serpent which is the devil and Satan, and chained him up for a thousand years. He threw him into the Abyss, and shut the entrance and sealed it over him, to make sure he would not deceive the nations again until the thousand years had passed. At the end of that time he must be released, but only for a short while.
Then I saw some thrones, and I saw those who are given the power to be judges take their seats on them. I saw the souls of all who had been beheaded for having witnessed for Jesus and for having preached God’s word, and those who refused to worship the beast or his statue and would not have the brand-mark on their foreheads or hands; they came to life, and reigned with Christ for a thousand years. This is the first resurrection; the rest of the dead did not come to life until the thousand years were over. Happy and blessed are those who share in the first resurrection; the second death cannot affect them but they will be priests of God and of Christ and reign with him for a thousand years.
When the thousand years are over, Satan will be released from his prison and will come out to deceive all the nations in the four quarters of the earth, Gog and Magog, and mobilise them for war. His armies will be as many as the sands of the sea; they will come swarming over the entire country and besiege the camp of the saints, which is the city that God loves. But fire will come down on them from heaven and consume them. Then the devil, who misled them, will be thrown into the lake of fire and sulphur, where the beast and the false prophet are, and their torture will not stop, day or night, for ever and ever (Rv 20:1-10).