The reason that God gave Isaiah thse messages about the downfalls of Egypt and Cush at the hands of the Assyrians was to take away the trust that people had in those ancient political powers. The residents of the Middle East found it easy to "fall back" onto these superpowers in times of crisis. But Assyria would take out both of these traditional strong countries. There would be nowhere to run when Assyria invaded Israel.
This reality was meant to fix the eyes of every Israelite upon The Lord. When the props of Egypt and Cush were kicked away, they'd find nothing to lean on. There could be no political alliance in which to trust. No institution of man would stop what God had prophesied through His prophets. The trial would come and nothing could ease it in any way.
Of course it is very unnerving to have all of your traditional resources emptied. And God warned His people well in advance that this was His purpose. He would do it. And they could only know that He had decreed it. The judgment was meant to focus them upon Him.