BX D&D has a curious relationship with plate armor.
At 60gp a suit, there's a really good chance most 1st level fighters will be able to afford plate, assuming they made a good roll on their starting money. Additionally, plate seemed to be the default fighter armor for NPCs in those early games , too. We'd kick off most campaigns with the Keep on the Borderlands, where just about every rank-and-file soldier in the keep is dressed in plate. It seems far too ubiquitous and democratic to be the cap-a-pie of the noble born medieval knight on a destrier. Later in the BECMI D&D line, "field plate" was added as a new armor type to represent armor reserved for the wealthy nobility. Nowadays, most of the retro clones and later games have upped the price on plate, making those 1st level fighters wait until they've survived an adventure or two before they upgrade to it.
However, I wonder how much of the assumed survivability for low level fighting types was contingent on AC 3 or AC 2 (or better) right from the first excursion?
It becomes an interesting question in light of the last post, where it was suggested the high middle ages of Europe might not be the best analog for the ideal adventuring society. When you shift the milieu to the Spanish Main or the Roman frontier or some early modern period, medieval plate mail no longer makes sense as an armor choice in the wider world.
I find myself evaluating two contrary positions. The rules approach assumes armor classes are absolute, and the medieval knight in full plate represents the armored pinnacle of personal protection. The unarmored musketeers of the 17th century are easy to hit, in game terms, and the legionaries in their lorica segmentata or lorica hamata would be ill-protected against a medieval knight as well. Most characters in a colonial-era game would be AC 9 in BX D&D terms!
Alternatively, you can assume the game assumes the premiere fighter of the time period should be difficult to hit, and come up with an alternate scale to allow fighters to achieve better AC regardless of the military technology of the period. Perhaps the fighter in the 17th century increases their base AC like a monk to represent the ability to duel or fire and maneuver with better skill? The heavily armored roman legionary in segmented lorica would be armor class 2 or 3, the pinnacle of the armored warrior of his time?
I fall into the first camp, where armor classes are absolute, and one of the side effects of shifting out of the Medieval paradigm and dropping plate mail as a choice, is that front line fighters will be less protected. In a game with muskets and firearms, cover and concealment and personal dexterity increase in importance, and that seems like the logical progression.