Did you check those parses? They're using unorthodox tactics / gearing specifically to rank on WoL. Doing this isn't easier than doing it the regular way, and can only be easily accomplished by overgearing the content and absolutely should not be used as an indication of the way things usually work in this game.
This. Flip it around.
Let's say you're an Arms Warrior. There's a fight that calls for a OT, but you don't want to lose DPS by having a dedicated OT. You put on a sword and board, get your holy paladin to spam denounce on the boss for crit immunity, and have the rest of your healers cycle externals. As soon as the OT-requiring section is over, you swaps back to your two-hander, get out of defensive stance, and go back to DPS.
Oh, no! You've just made my role useless
by using all sorts of strange tricks to make it possible!
Wait. No you haven't. You've managed to get someone to perform outside their role by doing many unorthodox tricks. The only difference is that this kind of unorthodox trickery doesn't show up on WoL as "TANKS DOING TOO MUCH DAMAGE!!!111" so people don't whine about it.
Yet, tanks were topping meters in 5 mans throughout most of cata. Nothing changed. The new vengeance makes it roughly similar in a heroic, but ridiculous in most of the leveling dungeons. "Want to do awesome dps while you level in dungeons?! Roll a tank!" bleh.
This not because "tanks do too much damage." It's a combination of two unrelated things:
1. DPS classes don't get their AoE attacks until later.
2. Fights don't last long enough for DPS to ramp up. Tanks tend to front-load damage to help with burst issues, DPS tends to back-load. Most low-level bosses are at half health by the time I have my S&D on my rogue, and by the time I'm ready to do a 5pt Eviscerate, the target's dead.
Ditch vengeance. Give 2000% threat if you need to ensure tanks will have threat (if you don't want threat to matter) or just make taunt a fixate and be done with it.
Speak as a tank, if they did this, I would quit tanking, because there would be literally no incentive to play. At all.
"Okay. No skill involved in holding threat, I just push a button and monsters can never lose aggro. Well, if I don't stand here and do my rotation, I die. If I succeed at my rotation, we might not fail."
Put it this way: Let's pretend they're changing mages. You now do zero damage. Instead, you apply a consumable "damage debuff" that increases the damage your party does to the target. This debuff requires constant upkeep via a complicated rotation, and you can't beat the enrage timer if it's not there.
How fun is your class now?
Active Mitigation is a carrot-and-stick deal. If we foul up our rotation, the raid wipes. If we succeed, the raid still might wipe, but at least it won't be our fault. There's the stick. The carrot is that we can actually affect the outcome
of the fight somewhat. We can pre-pot. We can better cycle cooldowns. We can, if we're careful, trade some survival for damage.
If you remove tank damage by making us hit for the same as a level 60 in quest greens with a 200x threat modifier, we lose the carrot. There's no positive impact we can have on our raid, only negative, and playing a tank becomes a thankless chore with no upside.
Before active mitigation, tanking was boring. The Rogues and Hunters gave you threat, your gear choices gave you survival, and there were fights where you could literally
go AFK for the entire fight. It wasn't high stress, but it wasn't terribly interesting. Now it's interesting, and we're involved, and it's okay because we have an impact. If you get rid of the impact and leave the stress, nobody will freaking bother.