There's always an element of time to money, so your argument and the parent's don't contradict in that the romans might have not been profitable in the near term yet what they created was very profitable in the long term

Mostly on the idea that as the Roman Empire expanded its currency was devalued by precious metal content. That sort of pattern signifies to me decline. I understand that the currency may have been just keeping pace with the efficiency of trade. It seems to me that if trade was as efficient as Rome was hoping it would not have collapsed.

Another way to look at it is that the empire ran into a labor shortage and contracted back to a sustainable size.