Sub-Roman Britain (Cavalry etc)

[Medicus matt]

the best that any ancient missile weapon could accomplish was either a bruise or a dent in the armour.

I'm sorry but that's just nonsense. Thin 'bodkin' headed arrows were in use by Northern Germanic archers in the 4th-6th C and one of those, delivered by even a low poundage (say 60 lb) single stave self bow like the ones found at Nydam would go far enough into all but the thickest of hide to cause serious injury. That's not a supposition on my part, I've seen it done as part of a series of penetration tests using pig corpses as bodies.

Mail lasts for a long time and is easily repaired (as attested to by the finds of mail with twisted wire in place of missing links). I'm sure that all those hundreds if not thousands of mailshirts didn't just get loaded onto a boat in 410 and get sent back to the fabrica.