Another primary consideration in introduction & eventual disappearance of Segmentata?

[Nathan Ross]

they were typically no longer crested, nor as ornate, but a solid simple metal helmet

Many later Roman helmets appear simple because they've had the decorative metals stripped off - the Theodosian code suggests that 40% of helmets produced by the state arms factory in Antioch were covered with gold or silver plating. Crests are shown frequently in late Roman art, and crest fittings have been discovered from late Roman archaeological contexts.

Helmets from the third century, like the Theilenhofen and Niederbieber types, are particularly developed, and provide far greater protection than earlier models.

As for ornate, compare these two: first century and fourth century!