I seems to me that Rome is more of a Rorschach test than anything else. Because there was no single cause that definitively ended the empire. Ask 20 historians to tell the prioritized story of Rome's decline, and you'll get 21 largely distinct tales. If you want to blame moral decline, you can. If you want to corrupt political systems, poor monetary policy, immigration, disease, war, social dis-integration (hyphenated only to emphasize the meaning), wealth inequality, or even self inflicted accidental poisoning - you can.

And the worst part of all is that even if we had complete and absolutely perfect information on the decades leading to the end, you probably still couldn't find "the cause." Empires fail long before they collapse - inertia. One can easily argue that the Soviet Union failed decades before 1989. But for anything so monumental, inertia was enough to keep the corpse of a system moving forward for decades to come. And in the time of Rome, when the speed of dictate was that of carriage, sail, (and on rare occasion - pigeon) - this effect would have been magnitudes greater, which is difficult to even begin to appreciate the implications of.

Perhaps the most interesting, and telling, thing of all is how empires towards their end tend to share not just one of the "symptoms", but many.