The Roman state was arguably much more modern than the medieval kingdom. It was highly centralized and funded through taxation (most of the plundering was already done by the early imperial period).
Not sure the Huns were the biggest direct threat either (unless we think that they are directly responsible for the Gothic migrations/invasions who were the ones who took over significant parts of the empire).
The "taxation" point is arguable since so much of it occurred in the provinces and basically amounted to plunder. Also, centralization is not much of a marker of modernity: the Ancient Near East had large centralized empires, but they were also similarly vulnerable to collapse for practically the same structural reasons; we just know a lot less about those times and places because the sources are so much more sparse and understudied.
Paradoxically we do know a lot more about some of the ancient Near Eastern societies than most states that preceded or succeeded them. There are still thousands of clay tablets which nobody really had time to read, while pretty much all Roman and Greek texts (and effectively all administrative documents) that weren’t copied during the middle ages are lost.
> provinces and basically amounted to plunder.
Also redistribution. The mid/late Roman state spent had huge taxes and spent almost all of it paying for its professional army almost all of which was stationed in the provinces.
As a consequence the Roman economy was highly monetized, long distance trade was widespread and different regions economically interdependent which again seems rather modern.
Also when talking about “plunder” in the ancient world it's almost entirely slaves not gold/silver or moveable goods. That had mostly dried up during the imperial period.