Another primary consideration in introduction & eventual disappearance of Segmentata?

[Urselius]

Yep, p. 110 - Modares "ordered his men, armed only with swords and shields and disdaining heavier armour, to abandon the usual fighting in close order." A very explicit statement that late soldiery fought in more open order when necessary, and abandoned their armour so as to be more effective in this type of combat.