If you'd just stated that the caster jobs are OP, then how is it ridiculous for players to stack them?
Why would increased MP costs and reduced MP pool be enough to solve the problem?
Why would a slow, tedious over 25 minute fight be any more satisfying upon completion than a more fun 10 minute fight?
If you added MP cost to every weaponskill, non-caster classes would be likewise nerfed by reduced mana pools, and without means to regenerate, meaning that casters will not only be the nuking jobs and classes, but also the outlasting ones.
I think you may see an increased sanity to your posting if you'd refrain from using the word "immersion" for a few lengthy posts.