Another primary consideration in introduction & eventual disappearance of Segmentata?

[Nathan Ross]

Show me any evidence that the Roman economy actually grew during the period we associate with its decline.

Areas of it certainly did. The economy of Italy was fairly stagnant, with so much wealth tied up in senatorial land and property, but Britain experienced a boom in the fourth century - evidenced by the number of large new villas built during this period. The east was prosperous too: some scholars put the peak of ancient economy in the east in the 5th-6th century. The empire generally recovered in the 4th century after the turbulence of the previous era.

The Roman state was probably wealthier in the 4th century than it had been at any previous point - the solidus (introduced by Diocletian or Constantine) proved very stable, there was a direct tax on gold that tapped aristocratic wealth (AHM Jones estimates that the aristocracy of the era had incomes five times greater than in the 1st century) and many industries were taken into direct state control, further adding to imperial funds.

If you go to Rome you can still see the ruins of the Baths of Diocletian and the Basilica of Maxentius (both the biggest of their age), the Circus enlarged by Constantine, the walls built by Aurelian and doubled in height by Honorius. In Trier you can see Constantine's vast audience hall; in Split you can see the remains of Diocletian's fortified villa, and further east you can see the massive land walls of Constantinople. All over Europe there are the remains of the monumental building works of late antiquity, many of them state funded and imperial owned.

Any state which can build on such a scale (and equip its troops with gilded helmets too!) is clearly not short of cash.