When Did The Roman Army Standardize Using Plate Armor?

[Hanny]

The armours constructed of weaker materials were made to be thicker and heavier. The result is that they all stopped the same weapons.

Odd way to look at it, a human can only carry a certain amount and be expected to perform well in combat, so plate replaced mail as it gave more protection than mail, but was more weight per sq metre of coverage, what did not happen was to get the same protection from mail, sq metres of weight protection, was to wear multiple layers of mail and become immobile.

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But, I think you have forgotten by purpose the real advantage of the Segmentata: the greater protection level on covered parts.

This is a myth that needs to be squashed. All types of metal armour - mail, scale, lamellar, segmetata, etc. provided similar levels of protection.

You already wrote "Nobody would wear lamellar if plate was available."

Its not a myth, its just your unable or unwilling to accept ballistic properties 
reality.