Part of the problem with that is that "the offensive tank" is way too broad of an advantage to give to any one tank.

Nobody really comments on it, but "the tank with the highest burst damage" and "the tank with the highest DPS at the end of a fight" should absolutely not, ever, under any circumstance be given to the same class, because those are two completely separate advantages to have.
Conceptually, balancing tank DPS is pretty easy:

- One class does extremely high burst damage and has the lowest overall DPS
- One class has the highest overall DPS but does constant damage with no ability to burst outside of party buffs
- One class is able to do moderate burst, and has an overall DPS in between the other two.

That's meaningful differentiation in class identity with respect to DPS, without painting in an overly broad brush and just saying "Okay, this class is the best at damage, everybody else is fighting over their table scraps".