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

[CaesarAugustus]

They didn't abandon torsion artillery (if at all?) for some considerable time after the introduction of composite helmets!

They progressively abandoned the ballista, keeping the onager, so they did not abandoned the torsion artillery, just the ballista (and generally the double torsion engine). For what read, they progressively abandoned the field artillery, that field artillery that would have been really useful in Adrianople, for example.

For the progressive abandonment of the ballista, I am mainly referencing Tarver's The Traction Trebuchet: A Reconstruction
of an Early Medieval Siege Engine. We can accept or not all the part related to the trebuchet reconstruction (we have the same issues we have for the helmet, multiplied by 100), but the anlysis concerning the abandonment of the ballista and the reason for this is really interesting:

But by the late 3d century, a shortage of good artificers once again seriously hampered attempts to produce and deploy torsion artillery.
The simpler, one-armed onager became the standard piece of heavy artillery as early as the 2d century, as well as the arcuballista, which
Marsden identifies as a nontorsion device.22 "The artillery legionaries of the fourth century could no doubt maintain their machines in efficient working order, but they were constructed and given major overhauls in imperial workshops (fabricae ballistariae)."23O nly in such workshops could the army maintain the materiel and trained men necessary to produce effective torsion weapons. No such workshops were perpetuated by the barbarians who overran the western Empire.

As for 'progressive decay', the late 4th century ruling in the Theodosian Code instructing the Antioch armoury fabricae to cover a greater proportion of helmets with bronze and gold sheeting suggests both a wealthy state and a very high degree of organisation.

As I said before (in my opinion!), the late Romans didn't somehow forget how to make old style single-bowl helmets, and did not lack money or expertise, therefore the new pattern composite helmets must have been as good if not better than the old ones!

For the army organization in the late empire, apart what just written, and ignoring totally officer gold equipment (that is another sign of the incoming problems), we can read about the social status of the empire in Géza Alföldy's The Social History of Rome. And the effects of this are evident in what happened following Adrianople, with the army unable to reorganize itself.
The organization of an army has not to be searched in the officers' helmet, but in the average quality of the soldiers one. And in the ability to support the army with the siege and field artillery. With the ability to substain the army in its campaigns and so on. Something that has been progressively lost.