Holy trinity.. Or maybe not.

[Immut]

God forbid someone take that 5k damage hit that would one shot every other player. Yeah, we don't need players like that. -_-
Of course people would take more dps if they could, but It doesn't mean you'll survive the fight. The monsters hit very hard and you need someone to take that hit and to hold monsters in one place to make dps positional requirements easy to meet. On those point alone it is very desirable to have tanks. Period. Even Titan-Egi was a tank in the Ramuh situation. Without the tank you're all going to die.

Why is it any time I suggest changing the status quo someone has to tell me I'm wrong because of the status quo? I'm talking about radically changing the design of the game.

Well, the purpose of a tank is to tank sooo....

Also, not too sure you are aware of this but each tank class has a tank "stance" that can be turned on or off, so tanks that know what they are doing are able to switch that stance appropriately in the fights where they are swapping. Sure, this is never going to push out as much dps as an actual dps class but what would be the point of a high dps tank class? Then you would see an extreme over abundance of people playing that class, because it would pretty much eleminate the need for real dps classes that do not have a huge pool of hp and defense.

Seriously, I don't get how people can completely miss the scale of the post like this.

You can already substitute a highly geared warrior for DPS in most content. People don't do it because the survivability of your DPS doesn't actually matter. They aren't expected to get hit by anything that actually does damage. With the current game design, the only difference between a dps class and a tank class is the sliding scale of defense vs damage and the ability to generate enmity. There is more difference in gameplay between a ranged dps and a melee dps than there is between a melee dps and a tank.

Imagine Testudo was a PvE skill, and then imagine a fight with many adds. Strong ones that don't die in two hits and too many of them for two tanks to reasonably handle. The PLD uses Testudo on the Dragoon, their survivability goes up dramatically and they're able to tank an add or two themselves. That adds dynamism to the roles. It also gives PLD a reason to show up, even if they're not tanking the boss, because it would enhance the survivability of the entire party for each successive tank, and it makes armor and HP actually matter for classes that aren't explicitly tanks.

The fact that each role in this game is so pigeonholed is the reason why fights need to be so mechanical and gimmicky.