Empires often rise to power to fulfill a divine purpose, but once that task is complete, they must answer for their own cruelty. Babylon served as a powerful instrument in God's hands to punish idol-worshipping nations. However, once God finishes using the empire for this purpose, Babylon's turn arrives to face the consequences of its actions [רש״י]. Severe punishment and retribution will fall upon the nation for its wickedness [מצודת דוד]. The total collapse of Babylon is a direct repayment for the immense harm it inflicted upon Zion, an act of justice carried out entirely for the sake of the Israelites [מלבי״ם].
This downfall will not happen in secret. The impending disaster will be a highly public event. The exiled Israelites living in Babylon, alongside other nations, will witness the collapse of the empire with their very own eyes [מצודת דוד, ביאור שטיינזלץ].
Typically, witnessing the defeat of an enemy is a rare privilege that requires a person to possess special spiritual merit. In this situation, however, the retribution is driven directly by the power of God. Because He is the one executing this justice, the Israelites will be granted the ability to watch their oppressor's ruin regardless of their own personal merit [חומת אנך].