ANSWER: When God prepared the earth during his six creative days and then finished his work with the creation of Adam and Eve, it was God’s intention that humans should live forever in an earthly paradise. But because of the rebellion that took place – first in heaven, when a righteous angel sought to steal God’s glory and then on earth, when Adam and Eve followed the rebel angel – God purposed for a kingdom to rule the earth for 1,000 years in order to bring all things back into subjection to himself.
And Jesus Christ was given the kingdom as a reward for his faithfulness to God all the way to death. And as Paul stated in the scripture referenced, as king after Jesus has brought all things into subjection, and the last enemy – death – is vanquished, then even Jesus himself will subject himself to God. At that point the kingdom of God ceases to rule and Jehovah resumes his rule as in the beginning, before the rebellion brought death upon the world. And in contrast to the usurper – Satan, the Devil – Jesus voluntarily relinquishes his authority in full submission and acknowledgment of God’s supremacy.
But in what sense is the kingdom eternal? Christ will rule forever in the sense that what he accomplishes will always endure. Never again will God allow any rebel angel or human or any other creation to slander him or challenge his rightness or dominion. And never again will it be necessary for God’s sovereignty to be vindicated, which is what Christ has done and will yet do.
Here is a link to a Watchtower publication on the topic