The Kingdom of God is forever! How could it be physically manifested on this earth since this earth is destined to be burned up with everything on it? If the Kingdom of God comes to this earth, then it cannot be unending and forever!
This verse speaks of the spiritual Kingdom of God in heaven. Believers ascend alive to heaven after our physical body dies, a living (spiritual) soul. Since the realm of God is not physical but a spiritual habitation, we shall then be the image of Christ, a spirit being or as Paul says we shall belong to a spiritual body in heaven. As on this earth we bear the image of natural, earthly man, so too in heaven we shall bear the image of the celestial/heavenly/spiritual man.
1 Corinthians 15:35-50 (KJV) But some man will say, How are the dead raised up? and with what body do they come? Thou fool, that which thou sowest is not quickened, except it die: And that which thou sowest, thou sowest not that body that shall be, but bare grain, it may chance of wheat, or of some other grain: But God giveth it a body as it hath pleased him, and to every seed his own body. All flesh is not the same flesh: but there is one kind of flesh of men, another flesh of beasts, another of fishes, and another of birds. There are also celestial bodies, and bodies terrestrial: but the glory of the celestial is one, and the glory of the terrestrial is another. There is one glory of the sun, and another glory of the moon, and another glory of the stars: for one star differeth from another star in glory. So also is the resurrection of the dead. It is sown in corruption; it is raised in incorruption: It is sown in dishonour; it is raised in glory: it is sown in weakness; it is raised in power: It is sown a natural body; it is raised a spiritual body. There is a natural body, and there is a spiritual body. And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit. Howbeit that was not first which is spiritual, but that which is natural; and afterward that which is spiritual. The first man is of the earth, earthy: the second man is the Lord from heaven. As is the earthy, such are they also that are earthy: and as is the heavenly, such are they also that are heavenly. And as we have borne the image of the earthy, we shall also bear the image of the heavenly. Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption.
Only our spirit ascends to heaven after our body has died, because our flesh shall not be immortal & incorruptible until an hour coming when the last trump sounds. And neither flesh & blood, nor corruption can enter there. Since our spirit became eternally alive when we were born again of His Spirit, our spirits go to heaven with eternal life and can never die. That is why Christ promised that whosoever lives and believes in Him shall never die.
The Kingdom of God now being built in heaven shall never be a physical Kingdom upon this earth. After this earth has passed away, a new heaven and new earth will come down. The living spirits present in heaven with the Lord will come back with Christ and be reunited with a resurrected immortal & incorruptible body of flesh, then we shall be complete living soul again.
Revelation 21:1-4 (KJV) And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea. And I John saw the holy city, new Jerusalem, coming down from God out of heaven, prepared as a bride adorned for her husband. And I heard a great voice out of heaven saying, Behold, the tabernacle of God is with men, and he will dwell with them, and they shall be his people, and God himself shall be with them, and be their God. And God shall wipe away all tears from their eyes; and there shall be no more death, neither sorrow, nor crying, neither shall there be any more pain: for the former things are passed away.
Of course the Kingdom of God in heaven, the same Kingdom spiritually within us is not yet complete. As Christ has said the Kingdom of God has come and we have been entering into His Kingdom through His Spirit since He has come. You call it a contradiction that the Kingdom of God is here but is not yet complete. Maybe I should say is not yet finished being built. When the last person to be saved is born and enters into the spiritual Kingdom of God according to grace through faith, then the spiritual Kingdom will be completely finished and the saints of Christ shall live with Him forever on the new earth where there shall be no more death, no more sorrow, for all things then shall be new again.