Regarding the Gladius and Mail

[Crispianus]

Indeed tin WW1 the h4lmet was designed to give protection from 'air burst' artillery shells exploding over the trenches.
America designed the M1 helmet to give better overall head protection and from 1943 the British were issuing the similar Mk3 'Turtleshell' helmet, it was used right up to the 80s.
It's one of the frustrations of modellers trying to depict Commonwealth troops in the latter half of the war  nearly all figure kits come with the Mk2 'Brodie' helmets, which were still used by some units and longer serving men weren't always issued new ones.
https://en.m.wikipedia.org/wiki/Mk_III_helmet

Its interesting to note that most later helmets already had test designs in WW1, particulary in the US but were dropped in favour of ease of production(huge quantitys need quickly), cost or simply because they looked too much like the enemies....
Cost and ease of production could be applied to some types of ancient armour just as easily, particularly when supplied by the state, however the later use of mail and apparant phasing out of segmentate may represent an attempt to rational the system with an armour type that whilst it may not be the cheapest to produce is easily repaired, long lasting, practically one size fits all, which could potentially make savings in the long run... it would also be difficult to damage it so extensively as to make it useless in combat, whereas a broken strap or hinge with segmentata could make it difficult to wear or move...

Just my thoughts on it not written in stone...