Countries, let alone empires, are very large entities. I'd argue that they fail long before they collapse. Consider the Soviet Union. While the final reason they collapsed was economic, it would be difficult to argue that it could have continued to persist indefinitely otherwise.

In my opinion the Soviet Union had failed decades prior to its collapse (which is saying something for a system that only lasted 69 years), the root cause being the loss of faith of their own citizenry. And in such a scenario, any bump in the road is suddenly going to turn into a catastrophe. I think blaming the bump is failing to see the forest through all those trees.

The Roman Empire spanned from Great Britain, across Europe, and even along North Africa. And the speed of light was the speed of horseback or sail. Things, even when done with the utmost haste, were slow. And so in this scenario the separation between the failure and collapse was going to be even greater.