Another primary consideration in introduction & eventual disappearance of Segmentata?

[Nathan Ross]

the Antonine Plague... is markedly understated as a cause of decline in the Empire... and it's after this period that Segmentata disappears... a blow from which it never recovered... that's why we view Commodus as corrupt... Roman and Greek education virtually disappear... and within 40 years you never see plate armour again.

You seem to be conflating a lot of different things here - the plague, the disappearance of segmentata, imperial corruption and declining literacy. I'm not convinced they're so easily connected!

The Antonine Plague was doubtless very severe, and the empire may have taken a decade or even two to recover. But under Septimius Severus (AD193-211) the army increased in size, and the empire reached its greatest extent. And the Arch of Severus in the Roman Forum appears to show that many of his troops were still wearing segmentata when they conquered Parthia.

The craziness of Commodus is described by contemporaries like Dio. If people at the time thought he was a bad emperor, his badness could not have been a product of the time, surely?

The idea that the post-Commodan age saw a decline in Roman culture is a bit of a 19th century myth, drawing on Gibbon, and on Dio's own melodramatic claim that the empire fell from an age of gold to one of iron and rust. In fact, the Severan era (right through to the mid 3rd century) saw a flowering of literature and philosophy across the empire. Styles changed in art and architecture, as they tend to do, but we no longer see that as a decline.

Education certainly did not disappear, although the literacy of the average Roman seems to have suffered. Many of the 'barracks emperors' of the later 3rd century were no longer drawn from the elite educated class - they were common soldiers risen to the top - and were naturally less well educated (Maximian was alleged never to have heard of Scipio Africanus!), but that does not seem to have affected their military abilities.

Constantine may have been the son of a soldier, and a bit of a drunk, but anyone who can compose and deliver the two-hour-plus Oration to the Saints is clearly both literate and educated. Constantine appointed Lactantius as tutor to his son, Caesar Crispus, just as Valentinian appointed Ausonius tutor to his son Gratianus; the virtues of a classical education were still accepted in the fourth century!