Another primary consideration in introduction & eventual disappearance of Segmentata?

[Nathan Ross]

I cannot find one source on the Antonine plague that doesn't say it destroyed Roman culture - and likely education and philosophy with it

I'd like to see one decent source that does!

The immediate effects of the plague were severe - how severe is very hard to judge - but the longer term effects are a matter for much debate. Richard Duncan-Jones's paper 'The Impact of the Antonine Plague' makes the case for high mortality; other scholars have questioned this (and there's a good summary of the debate here from Christer Bruun).

Commodus was unpopular with the senate, and had some strange ideas, but the empire did fairly well under his rule. 'A breakdown of morals and restraint' is a purely subjective opinion. It might be that the successive disasters of the Antonine era contributed to the rise of various sect-based monotheisms, including Christianity, but this swing away from traditional public religion may have had other causes, and was part of a much wider social shift.

For culture and social change in the post-Commodan era, I would recommend David Potter's The Roman Empire at Bay, and Swain's Severan Culture. Both contain plenty of evidence and debate on the flourishing intellectual life of the 3rd century (the 'second sophistic').

The Principate had great metal.

And the dominate had better metal (technology). Just look at the development of the Roman sword between the 1st and 4th centuries. Simon James's Rome and the Sword is good on this.

Meanwhile - I still don't see how declining literacy would make an army less effective. Nor do I see how this affects the end of segmentata, which as we've pointed out above continued in use throughout the third century.

Perhaps if you want to discuss the wider effects of the Antonine Plague you could start a new thread on that topic, maybe in the 'civilian' section?