You said no medieval footmam should wear medieval armour ever, yet there is clear evidence many of them did.
Not to mention the fact the plate armour itself, was a very bad idea on its own ...
It was never effective, because of firearms which could pierce it or all kinds of armour piercing weapons, maces etc ...
So eventually and pretty quickly was stop being used completely.
But in Warcraft you have new fantasy materials, like Mithril, Thorium, Saronite etc etc...
Which could make them much more effective then in real life.