Micah chapter 4 begins the same way Isaiah begins.
Who copied who?
It’s not necessary that anyone copy anybody. Remember: the prophets speak for God, not for themselves.
Micah continues the alternating judgement of God against His people: condemnation, then promise of glory. Then, back to condemnation.
It’s not that God cannot make up His mind what to do. He’s already done that. He just wants His people to understand that with their sure coming punishment, He is not done with them. Judah will not cease to exist. When He’s done with them, a small number, a remnant, will return, be blessed, and rebuild. They will not do so on their own, but by the foreknowledge, fore-ordination, and power of God.
Assyria, a nation known by Judah, will come and wreck havoc on her. And that will not be the end. Babylon will come too. To captivity she will go, but there will be a return.
Why does God keep dealing with the descendants of Jacob?
It would be this way with any people. Jacob is but a sampling of the world. God has bound up his reputation with that of His people. He intends them to be His light to the nations. They will either be an example of His righteous grace or punishing judgment. The choice has been their’s to make. The Church today should learn from the lessons here. We too are called to be a light to the world. What will the world see by our light?