The story of his ruin is simple and obvious; and, instead of inquiring why the Roman Empire was destroyed, we should rather be surprised that it had been subsisted so long. What argument is the author making in this passage?

The answer to your question is:
"It is a wonder that Rome did not fall even sooner."
Hope this helped. :)
Answer:
The answer is: "It is a wonder that Rome did not fall even sooner."
Explanation: