Another primary consideration in introduction & eventual disappearance of Segmentata?

[Urselius]

I don't think that your equation of widespread literacy and plate armour holds up very well. We know from archaeological remains that the 8th-9th century Turcic Khazars of the North Pontic Steppe produced plate armour greaves and pauldrons. Even with their conversion to Judaism, I do not think that they had any greater levels of literacy than was found in contemporary Western Europe, and westerners were exclusively using mail and scale at the time. Furthermore advanced metallurgy is not reliant on literacy. The illiterate or semi-literate Celts and Iberians had better metallurgical skills than the literate Romans, we know this because the Romans said so, and archaeology has tended to confirm it.