With the launch of Stormblood

[aleph_null]

Not really. MT and OT are designations which refer to which tank is actively tanking at a given point in time.

I don't mean MT as who's actively tanking at the moment, since there are forced tank swaps so it's kinda pointless to classify tanks as such. What I meant was classifying tanks as "MT type" or "OT type" based on their skillsets. Having those two types of tanks doesn't mean the "MT type" has to tank the boss from pull to the end of the fight, since as you've mentioned there are cases where the "OT type" tank can help the group just like doing unchained pull. Warriors being the better choice to pull the boss doesn't make it more of an "MT type" than pld/drk, in the end they just do a few combos before the pld/drk provokes the boss, since that'd benefit the group dps more by having access to shield swipe, reprisal and blood price.

Problem is, since every content can be accessed by a random party setup, you can't design content where one tank job can't MT. So, if all tanks can be good MT, all tanks should be good OT too.

Being an "MT type" tank (at least based on what I meant above) doesn't necessarily mean you can't OT, vice versa. It just means you benefit more by MT-ing. Currently warrior is really good because it's the only tank having access to slashing debuff. Let's imagine if all tanks have slashing debuff (or none of them have), pld/drk combo wouldn't be as bad as it is now. But if you have a drk/war or pld/war combo you'd still want to have the war pull, then the other tank provoke after a few aggro combos, since war still benefits less by taking hits compared to the other two. Balancing tanks such that they can at least do decent job (not optimal) in either position is good enough imo.

Well, right now, we have 3 tanks, and 2 clearly superior combinations. Create another OT (Still viable as an MT), and you'll probably end with only 1 superior combination.

Come on let's be realistic here. Even if we have 3 tanks that are good at both MT and OT positions, we'll still have one mathematically superior combination even if by a really small margin (or at least one superior combination for any given fight), unless you make some of them virtually the same.