Rev 17:8 The beast that thou sawest was, and is not; and shall ascend out of the bottomless pit, and go into perdition: and they that dwell on the earth shall wonder, whose names were not written in the book of life from the foundation of the world, when they behold the beast that was, and is not, and yet is.
Notice that the beast "was" before John wrote the book and "is not" at the time John wrote the book and then later would ascend out of the bottomless pit. How can that be describing a man who will eventually be cast alive into the lake of fire? That man would be thousands of years old by now. You're not recognizing that the beast and false prophet don't represent men at all. They are said to be cast alive into the lake of fire, but you need to remember that the beast (first beast) and false prophet (second beast) are symbols. They are symbolic entities being symbolically cast alive into the lake of fire. Just like death and hell are symbolically cast into the lake of fire (Rev 20:14). Death can't be literally cast into the lake of fire, right? So, the only way that your point here would be valid is if you could somehow show that the beast and false prophet represent living human beings, but I don't believe that is possible.
Also, the following supports the view that fire will be coming down upon the return of Christ, which would support the view that Rev 20:9 occurs at His coming and describes the same battle as Rev 19:
2 Thess 1:7 And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels, 8In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:
2 Peter 3:10 But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up. 11Seeing then that all these things shall be dissolved, what manner of persons ought ye to be in all holy conversation and godliness, 12Looking for and hasting unto the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat? 13Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness.