If something can't go on forever, it won't. Simple, right? And yet, the notion that things will always continue to trend as they are is a common fallacy, and leads to real estate bubbles and the collapse of monetary systems.
However, this is a relatively recent trend, and we forget how many ancient civilizations had sophisticated capabilities that have been lost to us, or that we have had to recreate from scratch. They rose, they fell, they lost the recipe. Historians continue to analyze why the great civilizations declined, but once large, complex internal structures form, they begin to differentiate into interest groups that no longer have it as their principal goal to perform their greater function, but rather to maintain or grow their share of the resources flowing through the machinery. When a system fails this way, we call this "collapsing under its own weight."