Jeremiah tells them if they return to God, everyone around will be amazed by how God will deliver them. But God knows they will not return. He tells them they will be destroyed, and the destruction will be appalling.
It’s easy to mistakenly believe God is saying “I will reward you for good behavior and punish you for bad behavior.” That’s really not what he is saying. He is saying “if you obey me, I can bless you and everyone will know I did it.” He can’t bless them when they don’t obey because no one would know which of their gods had done it. When we depend completely on God and let everyone know we depend on him, it frees him to work in our lives.