When Did The Roman Army Standardize Using Plate Armor?

[Nathan Ross]

segmentata wasn't replaced by anything; it simply ceased production.

But the Roman army did not, as far as we know, get any smaller during this period. So the troops formerly equipped with segmentata must later have been equipped with... something else. Therefore we can speak of it being replaced. I just don't think we can be sure it was necessarily mail or scale that replaced it.

it is quite hard to demonstrate that segmentata is cheaper than mail

The decorative hinges, brass edging, enamelled studs and washers that appear on some sets of segmentata demonstrate that this was not (or not always) cheap, poor-quality or poorly-made armour.

the centralized armories... more interested in fulfilling orders than in sending quality products.

Since we have imperial orders for the fabricae to produce large amounts of gilded and silvered helmets, they were clearly producing 'quality products'.

But the myth that the late Roman state was poor and its military equipment was cheap and substandard simply refuses to die!

Segmentata has been introduced following the battle of Carrhae

As there was at least a generation between Carrhae and the earliest known finds of segmentata, and this type of armour was far more commonly used in the west than the east (where the only find, I think - Gamla - was being worn by a western legionary) then a defence against Parthian arrows can hardly have been a reason for its introduction.