The reason Legions had mail armor isbecause of the mail armor endurance not quite it's effectiveness. Also, Lorica segmentata allowed a lot of flexibility even more than the Lorica Hamata as it was lighter. The roman metal work you talk, is inconsistent way after the lorica segmentata was abandoned. Also, if you talk about chain, I'll bring plate (the principle of lorica is plate armor) armor wich was the last tipe of armor and the most efective one. Also, armor in the late roman empire was ditched (not only segmentata but hamata as well being replaced by lighter tipes of mail or no armor at all) due to the increasing number of barbarians in the army and the change of tactics. This gave Roman infantry very small endurance while fighting heavy cavalry.
I believe in the end, Lorica Segmentata was eventually ditched because it's complexity and time consuming production. In order to support this, let me just mention the fact that the romans introduced several modifications to the armor in order to simplify it until they finaly ditched it. And Philipvs Vallindervs Calicvla, if you have three overlaping plates of 1mm (the ingenuity in the LS is this particular thing) gives 3mm protection. Gothic armor had 2mm. And, it was heavier. And missiles didnt work so well against Gothic armor. At least anything short of an arbalest or a mongolian bow with boadkin arrows. and still it might just not punch it.
Anyway, just my 2cents and sorry for the long post.


Cheers...