That's not what can't means.
Hardly what I said. Every Red Mage in Final Fantasy: a subset of Black Magic (Damage), a subset of White Magic (Not Damage), and decent melee weapons/armor (Damage). Red Mage is more DPS then it ever was Heal or Tank. Not open ended design, it's a set pattern. Defensive stuff were gimmicks in, really, just 11. That has nothing to do with how tanks survive.
If we're going to say what makes sense for Red Mage, it's not tanking.