As a tank, you often have to balance what you'd prefer to do against what a given group wants/expects you to do. For example, the reason that the str/vit gear choice became contentious was because it wasn't just about simply finding the correct choice for the fight and the group. It was also about catering to non-tanks' preconceptions on what that correct gear choice ought to be.

Presently, if you fill the MT slot, you're not just a PLD or a DRK. You're a PLD/DRK. There's a tacit expectation that you'll use whatever is optimum for an encounter (i.e. whatever was used in the world first clear).

For the entirety of ARR and the start of HW, the community felt that PLD was the de facto MT out of necessity because of it's high defense (it has a shield, it must be more defensive, right?) With the advent of A3S and A4S being cleared first on a non-PLD MT, DRK has managed to carve itself a temporary niche as an offensive-orientated alternative for MT (it doesn't have a shield). If you erode that too much, people will fall back on what's comfortable and familiar ("wait, you can't MT this fight on DRK!")

The challenge isn't designing these classes so that they are equally capable of tanking an encounter. The challenge is designing them so that the community perceives them as equally capable of tanking the encounter.