Food for thought:

Try these categories (formalized/nominal types A, B, and C into one of which each attack must fall) from any other perspective.

Take basic elemental damage, for instance:
Maybe an enemy is vulnerable to Fire and Wind. Okay, then you're forced to run WHM, SMN, BLM, and maybe MNK and NIN.

You can try to fix this by then aspecting a variety of attacks to various elements, but then you still end up reducing full toolkits or degrading their reward structures to just those particular attacks. NIN now has to spam Gust Blade and Aeolian Edge, Monk's damage lives or dies by whether Reply of Fire and Reply of Wind direct crit, DRG spams the newly wind-aspected Chaos Thrust / Chaotic Spring, etc. Whatever the case, you take something of which you had 100% useful... and reduced it to being useful some 16.7% of the time.
That waste, and subsequent loss of gameplay of need for extreme button bloat to return what we already had... is essentially what the suggestion would do to mitigation tools.
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Now, compare that to what can be achieved through what differences can exist even without any nominal categories dividing them (no magical vs. physical or STR vs. INT) -- namely via differences in duration, depth, scaling factors (and therefore synergies), frequency, and source of mitigation tools:
Enemies might have, for instance, a flurry of weak attacks with nonetheless high total damage, or singular strong attacks. Against the first, a duration of flat damage reduction is best, while against the latter, a single-hit high %DR is best.

But, they have additional potential synergies, too. For instance, one may pair the %DR with a flat reduction in order to take 0 damage, thereby preventing the attack from inflicting its debuff.

If you have an attack that steals Haste from an enemy, not only does that reduce their AA frequency or slow a flurry of attacks enough to be healed through, but it might also provide you enough hits to ready an extra active (gauge-sourced) mitigation/healing/suppression tool.

Etc., etc.
Such provides actual room for agency, rather than a mere color-matching pretense of complexity. Rather than playing Simon Says with a third each of a mitigation kit, you're rewarded for precise knowledge enough to minimize excess flexible mitigation spent in avoiding a debuff or in making an attack survivable if a latter one might not be without sufficient resources saved for it, for knowing not just what tools your cotank has available but also how they might synergize with, rather than merely supplement, your own..