Regarding the Gladius and Mail

[Robert Vermaat]

In the end, I think Segmentata was probably slightly better at saving people.  It just wasn't superior enough to warrant it displacing mail as the standard armor, esp. when mail has so many other good qualities.

Well, without of course the expletives (removed) and the 21st-c. analogy that's not particularly useful, isn't that what was said here in earlier discussions? Armour, the adoption  as well as the disbanding of it, may very well have been a case of fashion, cost, local availability as well as problems with construction and maintainance. Mail armour may in the end have scored better overall.
Like you say, nobody knows for sure.