Krugman cites it: If something can't go on forever, it will stop. True.
Even if something can go on forever, it will stop eventually.
Even if something can't go on forever, it can go on for a very long time.
One might say the difference between these corollaries and Stein's original is the difference between a real-world, historical approach to macro and the equilibrium approach of the mainstream. Fresh or salty, it's all water "when the storm is past and the ocean is flat again..."