As Shurrikhan details to some extent, your fix misses the main issue with PLD almost entirely.
The reason why WAR is good is pretty obvious. They're good. Period. It doesn't matter what situation you throw them into, they're equipped to deal with it. This applies to their class and essentially all their skills. They're all good skills in any situation. Outside of some QoL issues, they're flawless.
Why is DRK the other preferred tank atm? Because within the current content, they fit that same mold. They're good. Period. Because of the way fights are designed, DRK's strengths are very apparent and align in a synergistic way with the encounters. At the same time, those fights make much of PLD's kit much less effective.
PLD can still be a strong job. They're better than DRK in content where their strengths are utilized like Thordan EX. The entire problem with PLD is that they're forced into a niche due to how situational a lot of their skill-set is. Your suggestion does not fix that, it only makes it worse and is only going to restrict overall design space.
There are two solutions but one singular direction in addressing the issues PLD has. Design content to balance the three tanks or design all three tanks so that they're balanced regardless of content. The singular direction is that all three tanks need to be equally viable in all current content in any mixed pairing.
To prove a point, let's just be stupid for a second and give PLDs the same stack mechanic that WARs have. Bulwark now generates a stack. Tempered Will now generates a stack. Using Clemency now generates a stack. Cover now generates a stack. Hell, we're not even done with all the situational garbage PLD has but let's just stop there for now. Give PLDs weapon skills equivalent to Fell Cleave, Decimate, IB, Unchained, Steel Cyclone, etc. Instantly, Bulwark is now usable in any fight and in any situation. Tempered Will and Cover just went from relevant in 1 fight in all of Gordias to being an ability you use on CD just for the stack. Clemency is now no longer such an intolerable DPS loss because you get a stack. Now, you can throw PLD into A1S or any other fight that is predominantly magic damage and their skill-kit would still be usable and relevant. Now you can design any fight you want to and bring any tank you want to (though DRK would obviously need some love, too).
The idea is not to separate the tanks so that if you want to be the best tank player possible you need to level and gear all three. Or, if you want top progression, that you need to bring the class instead of the player. Pushing tank design into niches is a horrible, horrible idea that is both stupid and unhealthy for the game.