Available mitigation dictates how outgoing damage gets tuned, not the reverse, since all tank combinations need to be able to clear content. Discrepancies limit your ability to create challenging mitigation checks. As an example, if you release a big physical damage raidwide like HH in P10S, you have to still design to allow a DRK/GNB composition to clear on week 1, knowing that they will only have access to Reprisal. That in turn results in a softer check for a WAR/PLD setup, whereas a level playing field would naturally result in tighter checks across the board. The end result of all those supposedly 'little' advantages result in WAR having a smoother ride under all conditions.

I'm all for tanks having their unique strengths and weaknesses, but I don't really see a lot of give and take in that regard. When is WAR supposed to be at a disadvantage?

I think what needs to happen is that they need to get fresh eyes to look at the tanks from the ground up and re-evaluate how they can make a fair playing field for all involved. I think that there's a belief amongst the dev team that if they at least make WAR artificially popular, they can make people want to tank. This is shortsighted and will drive players away from tanking in the long run, simply because even non-tanks are recognizing and openly talking about the bias now. Nobody wants to invest in a role where they're forced to play a specific job to have the maximum advantages.