It really is an easy exploit... It's easy, but boring...
What you just described is a flaw in the AI not really the system itself.
Magic armor is a good thing, but only to mitigate magic dmg, CC shall have their own resists. It's way too easy to CC someone, once you destroyed their magic armor.
So the solution is to make sure that mobs and players have a way to somehow replenish their magic armor/physical armor; perhaps add in a defend option that regenerates a portion of it or lean on the usage of consumables.
