Armor: How effective was Chainmail

[Condottiero Magno]

There are, however, a number of high Medieval accounts of couched lances penetrating mail. Also, as has been already mentioned, mail often merely converted a piercing or cutting blow into a blunt-trauma injury, which could be incapacitating or mortal.

We've done this argument before in the Leather vs. Metal debate. Nothing short of a Couched lance can penetrate accurate mail, not even arrows. The couched lance was introduced by the Turks in the 9th century and needed Stirrups to be used with effect, so the Romans didn't have to worry about that.

Scorpiones can punch right through mail, I've seen video of a bolt going through the shield, armor, out the back, and into another guy's shield.

The couched lance wasn't introduced by the Turks and had been in use before the introduction of stirrups. Mail could be penetrated by piercing or cutting, depending on the situation, and if its wearer was so invulnerable, especially against most projectiles, why was it gradually replaced by pieces of plate, such as brigadine, similar to a variant of lamellar armor, and relegated to protecting armpits?