Another primary consideration in introduction & eventual disappearance of Segmentata?

[Urselius]

As for the 'lightly equipped' bit - it's an assumption, yes. Although I think, as you've said yourself, that we see a lot more flexibility (both tactical and strategic) in later Roman forces. Some equipment changes may have been directed by this need for relatively rapid movement over long distances (the Danube to Egypt or Mauretania, for example), far from reliable supply and repair opportunities.

There is a statement somewhere in Zosimus about soldiers being commanded to fight without armour. I think Elton mentions it in his Warfare in Roman Europe book in the section on armour, around page 110.