If PLD had the same DPS as DRK, DRK would still be taken for 3/4 floors because of its better/more frequent magic cooldowns and ability to reduce raid wide magic damage and allow nin/drg to be used.
WAR and DRK have superior utility through raid wide damage reduction. This is what PLD is supposed to have on its side as the "utility" or "defence based" tank but currently it simply doesn't. Veil is lacklustre due to its cooldown, clemency is far too slow and/or expensive for what is - little more than a basic cure. Cover is incredibly useful but this is far outweighed by the fact it is only useful at very gimmicky times.
Hallowed of course is often bought up as utility and is job defining but it is usually pidgeonholed into filling in for a gap in cooldowns because PLD cooldowns are generally longer than the other two jobs, at least useful ones for current content (See; first week A1S PLDs having nothing left for the final buster, DRK and WAR don't even need their invincibility cooldowns as vengeance/IB and Dark mind were always up)
Let's not even go into WARs slashing debuff.
If tank busters were physical or auto attacks weren't trivially weak/infrequent (seriously more frequent boss AAs SE, both for shields and blood price) PLD may be considered to have a slight edge over DRK defensively but the superior offence and AoE prevention mean DRK still has something on its side. WAR meanwhile barely even notices the change because it's buster cooldowns don't care about damage type.
For current magic based raids we have:
DPS
WAR>DRK>PLD
Utility
WAR>DRK>PLD
Buster Mitigation
DRK>WAR>PLD
PLD is currently in a sad situation where it is bottom in mitigation, DPS and raid utility for most of the floors. Partly because of the prevalence of magic damage, but making busters physical still leaves it behind in 2/3. This seems fair though, 3 tanks, 3 rough areas in which they can specialize, have one excel in each without having to differentiate by damage type.
Having one magic and one physical tank is absolutely pitiful design when your bosses only use one type of buster, one of the two is GARUNTEED to underperform every raid cycle, this time round it hit hard because the class that under-performs doesn't even have the damage to reclaim some of its lost ground. Flatten/Flare breath was a much better design.