What are you talking about? The fall of Rome involved the loss of a huge amount of technology and learning, as people abandoned the idea of specialization of labor and cities so they could go work as serfs in the fields for feudal lords. For the citizens of Rome (not just the ruling classes), it was absolutely a large decrease in living standards, and in overall civilization. There's a reason the period following is called "the Dark Ages": there was very little education any more, and no one wrote anything down like they used to, so that period is largely a mystery (relative to how much we know about the Roman times). It took 1000 years for western civilization to get anywhere near the level of civilization that Rome had developed.

Now you're right that it wasn't exactly a "crisis for humanity" because it only affected one part of the world--western Europe (the eastern Roman Empire continued), and also didn't involve a massive die-off, just a regression of civilization in that one area.