Roman helmets: Imperial Gallic/Italic and Ridge - comparisons and sources

[Dan Howard]

Mail never stopped being used. The Romans used it before, during, and after the period in which segmentata saw use. Segmentata was peasant armour; the equivalent of munitions plate only worn by those who could not afford anything better such as mail or scale or musculata. There is not a single depiction of an officer or NCO wearing it.  When Diocletian reformed the military, he got rid of segmentata all together preferring to standardise mail armour instead, which is far superior in many respects.

This might help.
https://myarmoury.com/feature_mail.html

The Diocletian reforms led to an improvement of Roman military equipment, not a degradation of it. The equipment of this time is arguably the best they ever used. The steel used by the military of this time was superior to that of the past as well. Military equipment was completely standardised, rigidly centrally-managed, with strict quality control procedures. Goldsworthy's comments simply can't be substantiated by the available evidence.