Point is, PLD can get all the defense in the world but unless DDs can use their full strength on the boss, PLD will remain inferior.
We had an acronym for this in FFXI - it was "ONP." Over Nuking ______, you can probably figure it out.

If PLD gets proper damage mitigation you add the possibility of replacing a second WHM with another DD, making it more efficient all around. I understand you're worried about hate, but there should be more strategy to the game than "everyone do as much damage, as fast as you can, all the time." If that's how you want to play (and often, I want to play that way too), tank as a Warrior. But that doesn't mean PLD needs to duplicate those tactics to be successful. PLD can be slower if that's what it takes. But it should at least gain extraordinary survivability for that sacrifice.

I'd take a revised PLD as a "safety" option with people throttling back as needed, leave WAR how it is, everyone is happy. The problem now is PLD has little "endgame" use, giving them proper damage mitigation would help fix that while preserving WAR as it currently is.