This is ironic because there is no "almost everyone," just a handful of clueless D&D nerds who got it from reading rulebooks written by other clueless nerds. Anyone who's actually gotten their heads out of the fantasy novels and worn armor or seriously studied its use won't be saying "plate mail."
Excluding the fact that I said Platemail, chainmail, and all that long before Dungeons and dragons... and it was included in a "Historically accurate game" is odd... Of course that said historically accurate game was sorta odd... You could flatter people up at the begining of the game with kind letters... but everytime you did it, it had less effect (And even could have a negative effect)
I miss it...
The point is ignoring everything else that has been said: I like Platemail, Chainmail, Scalemail, and Ringmail... I can even go as far as to include Bonemail, Splitmail (Diablo woo), and shadowmail. Me and my friends all know what something is when I say "Mail". When I go onto forums and I say "Platemail" everyone knows what I mean. This game isn't likely to get non-gamers who never heard of mail... as as long as games continue to become more and more popular...
I really don't see the harm in doing otherwise...
Ps. Luckly I saved myself by saying Modern Niche Term! I try to save myself from "Counter points" with these Technicalities. That way when someone argues against them... They hit my counter traps which you not only hit... but downright said was true! Hurray for the word Niche!