16 wheels drove 16 mills. Enough to feed the entire region.There was natural climate change, a cooling, which (as such things had in the past) resulted in a pulse of migration out of central Asia, into the great kingdoms west, south, and east. And the Plague hit multiple times. Also, the Roman Empire (or so-called Republic before that) never had a public education system, a postal system, a banking system (other than private lending, which bankrolled everything), never got rid of slavery, and took far too long to devise a uniform system of succession (and when it did, it didn't last). If anything, corruption kept the Roman Empire alive until 1453, when the Turks took Constantinople.
The culprit for the death of roman civilazation these days is widely thought to be the muslims. They completely destroyed the roman civilization and tech on the south side of mediterranian and spain. that generally known and accepted. recent research has shown that muslim raiders flattened every coastal city on the north side of the mediterranian between 700-1000 ad.
Aside from all that, what have the Romans ever done for us?