Err ... as a rule of thumb, D&D has nothing to do with reality at all.
Plate armor was very widespread in late medieval times, and during the renaissance.
Some plate armor was of course created to insane standards, and super decorated, like the replica one Adam Savage from Mythbusters recently tried on.
But professional soldiers of the time could already afford a full set of plate armor. You wouldnt need to be super rich.
Even regular foot soldiers especially in german regions are shown as already wearing a plate harness, even if they couldnt afford a full set of plate armor.
