Another primary consideration in introduction & eventual disappearance of Segmentata?

[Robert Vermaat]

This is also the time the legions are the best payed and when tax collection was at an all time high.  I've seen figures showing that the Empire's income shrunk to just 1/8th it's peak (which was ca. 1st C).  

 
Please show me that the period in which the segmentata is used, is also the period in which the legions were paid the most (as you say) and that taks collection was at an all time high (as you say). 
As to the former claim, I have reservations because we know by no means all the details about military pay, and what we know seems to point at an alteration of payment in coin and payment in kind. Expensive equipment such as the segmentata (your claim) supplied by the state does not seem to coincide with an equally high pay, which was to be used to procure such equipment privately. Most of the time it's either/or, not and/and.
As to the latter, I would love to see figures for the Roman tax collection. Do we even have these? I fear that your claim is based on assumption, but I love to be proven wrong by evidence.
 

Show me any evidence that the Roman economy actually grew during the period we associate with its decline.  Everything I've ever learned, read, and seen contradicts that, so I'd love to be shown otherwise.

 
Show me evidence that the Roman economy actually shrank. Or better, show me hard date about the Roman economy in this period and the periods before. Although my assumption would, like yours, be that the Roman economy shrank during the 3rd, 4th and 5th centuries, I have no method to gain hard data to support that hypothesis. I would love for you to produce these, as you ask for hard data in return. I won't accept evidence such as 'look at all the ruins' btw.

The appearance of the Roman soldier, on both imperial monuments and individual tombstones, changes radically during this period. A mobile force would perhaps be a more lightly equipped force, but it was also cut off from the traditional legion armoury at home base. This might explain the shift from segmented armour, for example, which required skilled craftsmen to repair, to the more versatile mail, scale and even musculata (breastplate) type armours.

 
Why would you assume a mobile force to be a lightly equipped force? Are you not confusing strategic mobility with tactical mobility? A mobile force can march great distances, accompanied by carts that carry the heavy equipment (as the late Roman army seems to have done). That there was no ''traditional legionary home base' anymore was replaced by regional production centres, which supplied the army wherever it was stationed.

Well, it seems like you are going over to the dark side the other arguments in this discussion! This is exactly what was argued at the beginning of this discussion: segemnted plate armour was phased out because of production methods, not because society was crumbling due to disease, loss of literacy or massive drops in population levels.