That doesn't work for a few reasons.
First, resistances work different from armor in terms of how they mitigate damage. Armor mitigates a static percentage each time; resistances cause you to resist a varying amount (for non-binary spells) or either all or nothing (for binary spells).
Second, stacking resistances is something that is available to every class in the game; stacking armor is not (i.e. cloth can never get better than cloth). It is amazing how much of a difference 100-150+ resistance to a school of magic makes in a fight.
Third, almost all physical attacks in this game are instant. The majority of spells require cast times, can be interrupted, you can't be moving, can be silenced, etc. Therefore it is far more difficult to get a spell off than a melee attack. In addition, magic attacks require mana which is not regained (in any appreciable manner) as a fight proceeds, unlike rage or energy. Given their lengthy cast times and the relative fragility of casters, a resisted spell (especially after you've spent 3+ seconds casting it) is often deadly.
So I must disagree. I do not think you have reviewed exactly how resistances work as a game mechanic and how fighting with someone with 300 resistances as a caster is far different than fighting someone decked in plate armor as a melee class.