Thats my point.....until near Renissance, making plates were hard. Plates are still plates. I am pretty sure Medieval Full Plate is made up of small plates as well. Even there smaller plated breastplate the "Coat of Plates"(small plates) didn't come to Europe till the 1200s. Why? Cause produce sections of plates are expensive. Chainmail is cheaper and easier to maintain. Do you know the engineering capacity it takes to take a section of metal(very expensive) and fold it to perfect shape and articulation? Small plates are still plates. There is a reason Lorica Musculata got phased out, it was more expensive than Chainmail.
Seriously if the Segmentata was truly cheaper, the use of it would be more widespread. If I was barabrian peasant I would choose unconfortable over getting pierced by an arrow in leather arrow. Somebody mentioned how it was "cheaply made" by unskilled manufacturers.....hmm if that theory was true everyone would be wearing it!
You honestly think this is cheap? God the engineering capacity to curve those plates and make it perfectley fit over eachother.