The basis for the exclusion of firearms seems to be pop culture assumptions about medieval warfare.
Firearms were a part of medieval warfare. We already have the armor and weapons of the era when firearms were used. Also, there is an unused wpf slot for firearms already coded in the game.
The arguments used for the exclusion of firearms can thus be applied to ANY item type, making it a purely arbitrary and nonsensical.
Plate armor should be more protective than it is now (against swords and other weapons which could rarely penetrate it), but firearms are of course the perfect counter to plated soldiers. Firearms are not as cost effective against fast, lightly armed soldiers; here, bows will be more effective due to their range and firing rate.
The more knights the enemy brings, the more guns you bring. Reality has its own balance, but this is constantly forgotten by the anti-realism league.