With regards to your first point, I'd say that "actual correctness" did not feature female breastplates in any significant numbers for anyone to say that there was a historically accurate field model ideal to which to aspire. (And no, I'm not familiar with any such arguments, nor have I ever seen any.)
I meant that the correctness is not merely political. Sexism does actual harm, is not merely a meaningless political checkbox.
As for the field model - the same argument works the other way, though. When there are *no* extant examples of historical armor of that form worn into battle, why then deviate from the forms you do know existed?
Oh, and 'Imad ad-Din, was a historian of the Crusades. He did describe some women warriors:
"On the day of battle, more than one woman rode out with them like a knight and showed (masculine) endurance in spite of the weakness (of her sex); clothed only in a coat of mail they were not recognized as women until they had been stripped of their arms."
(The "only" should probably be taken with a grain of salt - helmets, footware, and gloves were probably also involved. )
Moreover, the fact that such a design is dangerous for the wearer is demonstrable with some basic thoughts of engineering.
http://www.tor.com/blogs/2013/05/boob-plate-armor-would-kill-you
With regards to the rest of your post, since it delves deeply into politics, I'm extremely surprised you brought it up, knowing that I can't respond to it. (Perks of being a moderator, I assume.)
Nope. We've had threads on sexism, as it applies to gaming. Morrus even had a series of videos with interviews on the topic not too long ago. My comment is within bounds on that basis.
This goes to the good faith argument. To state 'dragonboobs are sexist and therefore wrong' you have to assert that they influence culture more than they are influenced by it.
The road to heck is paved with good intentions. It is possible to bring a wrongness into the world, even when acting in good faith - you can make a mistake, you can not fully consider the ramifications of your actions, and so forth. The result is still something that should be corrected if at all possible, once you realize the problem.
You may try to make the argument that since the books do not really influence culture at large, there's no point in worrying about it in gaming books. But, I think that's a cop-out. It kind of amounts to, "Since I am not a mover and shaker on the large scale, I can be a jerk!"