Both are active mitigation effects, yes, but the way that they work is fundamentally different. Given that you'll normally be using them on a Tank to help soak a TB, Intervention wins out because it can be more powerful an effect with combination of other CD's. Whereas TBN is only ever 10% of DRK's max HP (Which will essentially be about equal to the other Tanks. Outside of WAR whom has HP boosts)
At base Intervention is still as potent as TBN (Multiplicative DR stacking aside at least)
Except this is how you unbalance things. For a long time WAR had higher DPS to account for lack of utility. How well did that turn out? Considering how you like to complain about how OP WAR has been for so long, you, of all people, should know how dangerous giving a Tank DPS in lieu of utility is for balance.
For all intents and purposes it is Holmgang that makes WAR the best MT. Defiance while defensively strong, is rarely used. Such is why people often mention that DRK is the current king of self mitigation because TBN is pretty effective in the meta where Tank Stances aren't used outside of the pull.
It's honestly not hard to figure out some things. If I wanted to spend the time, I could even go through ability by ability and adjust numbers to achieve balance. I'm just not getting paid to do that.
Except there's much and more that can be utilized to create different feels.
For a start, DPS rotations are ALREADY major differences to playstyle. With WAR being all about maintaining Storm's Eye and gaming their Beast Gauge. With PLD alternating between maximizing Fight or Flight and using Requiescat. With DRK being all about managing MP to use Dark Arts to empower skills as well as weaving in oGCD attacks like Carve and Spit and managing Blood usage.
Outside of that, there can be different flavours of the baseline CD's. For example, how Vengeance has the retaliation damage on it. How Sentinel is stronger but has a longer CD. So long as the baseline effect is the same across the board, tanks will be balanced even with little bonuses for flavour.
Then there's personal utility skills. Things like mobility from Plunge/Onslaught (Vs Tempered Will). There can be something like Interject/Low Blow that provides Tanks with a universal interrupt skill that ignores diminishing returns, each with having some class flavour (For example, DRK could get a shield if interrupting a spell. PLD could get a guaranteed block.)
Things that aren't part of the "Core" tank role are flexible to be made unique or to be given in a variety of flavours to suit the Tank(s) that get it.
Well, this implies that the balance team is as invested and knowledgeable about Tanks as players. Often this is not the case (See: How literally every WHM saw how terrible the Lily mechanic was from the reveal and how years later, SE still haven't got a clue as to why the mechanic is such garbage)
These are are actually more complex to balance, because their effects are of different strength.
It's why I don't particularly like them, because they're always going to be a nightmare to balance because of how vastly different each effect is for the same outcome (Not dying to a Tankbuster irregardless of the amount of damage it deals)
Cover and Intervention allows for a party to bypass some Tank Swaps. Since they don't need to actually swap the tank for the damage to be diverted away.
Meanwhile, TBN actually benefits you from tank swapping, because then it becomes 20% max HP instead of 10%. But this means that you're actually following the intended mechanics.
No, but you can get things pretty close, to the point that only hardcore min/maxers actually care about the differences.
Which is far better than forcing people to play specific comps because you can't be bothered to actually look at the issues behind the role.
The thing is, there's so much more balancing that would need to be done in order to make sure that you actually have a "MT-OT barrier" and then in that barrier then also balancing the pairs so you don't just have WAR + PLD dominating the game still.
Meanwhile, as mentioned previously, there's not actually a whole lot that needs to be done to balance out all Tanks between each other, it's mostly just sorting out the glaring inequality between the tools they have. Simple.
Since, it shouldn't even erode their identity. Unless you're telling me that outside of the 1m lower cooldown on Holmgang and existence of Shake it Off that WAR and DRK play exactly the same?
