with an expansion they can change everything, that the point of an expansion generally, add new mechanic that the current game can't support.

let's take one second and say that red mage will be a tank, i don't say it must or will happend but just give an example, using a spell for enhance himself for defend against enemy and enhance him attack for generate more damage (work nice with the En- of the red mage this by the way). they decide to add a new type of armor for the melee caster, at that moment it have no interest, but there come later a new expansion with the blue mage and some other caster melee that will need this armor.

yes actually a caster tank seems weird, but if we only look to the past we will not progress that the point.

and one more time Shippuu your idea of the sam is only a war with a different mechanic for replace the rage and that use a katana... *shrugs*

ps: add a tank jobs physical wearing a heavy armor and using another slash weapon will never arrive can we stop to beat the dead horse it become gross.