Another primary consideration in introduction & eventual disappearance of Segmentata?

[Flavivs Aetivs]

the loss of north Africa to the Vandals, would probably be my choice for the point after which revival was impossible.

This.^^

Even with Aetius the Western Roman Empire did not have the financial or military capacity to retake it (and he tried, twice in fact: 440/441 and in 453 with his invasion of Spain which would be followed up by his lackeys Avitus and Majorian.)