Why not rebalance all Job Roles based on Special Stats & Abilities?

Like for Tanks:

Give Paladin the Best Defense for itself & others, but lacking in Physical Attack with some moderate Magic Attack Damage.

Give Warrior the Highest Physical Attack with single hits & AOEs, but lacking Defense & Healing Buffs (much as I like them right now), so they would have to work in tandem with Healers more often.

Give Dark Knight the highest Magic Attack with some low Physical attack.

Make Gunbreaker the Jack of all Trades, but master of none (low-moderate-high based on Critical Hit for Damage, and Healing/Buffs tied to other stats that affect their buff timer & Healing numbers).

==========

White Mage:

Highest Magical Attack & Healing in the game, but lacking in other stats & MP Pool, and it would be more susceptible to KOs if not careful.

Scholar:

Low Magical Attack, but moderate buffs & Healing.

Astrologian:

Best Magical/Physical Attack buffs in the Healer Roles with low Damage & low-moderate Healing, but it should get re-worked to bit tbh as it works now.

Sage:

Best Physical/Magical Defense Buffs, and with some low-moderate Damage, but can attack faster than all the other Healer Roles.

=========

(so I know some of these classes already kind of work like that, but maybe put more emphasis on it)?

Because square refuses to make it so every comp can’t clear so in your example PLD trades damage for more Defense, but everything has to be scaled to ensure comps with no PLD still have enough defence to clear so then PLD’s defence increase becomes useless and it essentially just does less damage for no reason

Speccing the classes this way requires people to actually accept they may need to change classes throughout a tier

FFXIV's combat is not designed in a way that would make this work, or even feel good. Many, many aspects of combat are binary pass/fails. Take, for, example, any given raidwide in an ultimate, such as Final Chorus in DSR, which already easily leaves caster and healer jobs at around 10% HP when it hits. If white mage had less defense or HP than other healers, like you suggested, the combat designers would need to take one of two directions:

1. Tune Final Chorus's damage so that a WHM can survive it with the expected amount of party mitigation, making it feel undertuned to any comp that doesn't include a WHM, or
2. Tune Final Chorus's damage to be dangerous to the other healers, meaning it will outright kill a WHM with a high enough damage roll. Now either the overall level of mitigation needed is increased, or players need to give WHM single-target mitigation, which would be an annoying hassle in content like ultimate where you are frequently taking party damage that chunks people for 90% of their HP.

The only thing the kind of design you're talking about does is causing massive headaches for certain jobs. Nobody likes it when your job is locked out of party finder because it's determined that its "special" stat spread makes it ill-suited for whatever content is being run. It's the same for a tank who gets barred because it's bad at mitigating the type of damage the boss uses more often, or the healer that isn't brought because it's "lesser healing" doesn't cut it for the heal check mechanic, or the DPS that gets locked out because it's extra comfort utility doesn't make up for the fact that it's lower damage makes the tight week 1 dps check even tighter.