In chapter 39 Isaiah spoke of the coming judgment against Jerusalem. The house of David will be taken into captivity into Babylon, and the treasures of Jerusalem will be carried off.
Isaiah is the prophet who has said over and over again: "I will deliver you and this city" (38:6). "I will defend this city to save it, for my own sake, and for the sake of my servant David." (37:35) "Behold Zion...an untroubled habitation, an immovable tent, whose stakes will never be plucked up." (33:20)
How can it be that now he speaks of the exile of Judah?
Isaiah chooses not to explain it directly. Instead he turns to the future. Because indeed Zion will never be plucked up. Unlike Babylon and Edom, which will uninhabited and desolate "forever," Zion will endure forever. That does not mean that the exile will never happen; it means that Jerusalem will come through exile into glory...