TBN is an Infuriate that costs a quarter of our mana and has a chance to fail based on the rotation of the boss, while providing a shield that is overtuned at the expense of having a well-rounded defensive cooldown suite.
Its mitigation is fantastic. The rest of our cooldown suite is garbage. We would rather see TBN scaled back and the rest of our mitigation buffed.
Right now, as a DRK, you don't ever, ever want to tank the boss, unless its for a tankbuster, and then after that tank buster, you want your co-tank to take it right back. We have literally zero cooldowns that can be saved for fluff damage, cleaves, raid AoEs... everything must be saved for TBs, except TBN. And TBN is a horrible cooldown to use in between tank busters. We literally have nothing but that for 70s out of every 90s. And TBN is designed to pop quickly. It is designed for you to go back to taking FULL damage from the boss in less than 5 seconds. Do we really need to be told why this is bad?
It also is not 20% mitigation, it is a 20% shield that is gone within seconds.
Compare this to say, WAR for instance, who can Rampart (20s) ToB/Conva (40s), RI+Awareness (60s), Vengeance (75s)... and at 75s, Rampart is almost back off CD... You're covered by some cooldown or another for 75s out of every 90s. And this is out of tank stance. With DRK, you're covered for a second or two out of every 15.
TBN is also never enough to mitigate a tankbuster on its own. You must always pair it with either Rampart or Shadow Wall, so you don't even get to use those outside of your TB rotation. The great irony is that an ability that screams "USE ME ON A TANK BUSTER" is not strong enough that you won't have to save your other CDs to pair with it, but has a cooldown so short you're forced into relying on it in their stead for fluff damage. And fluff damage DOES matter, particularly when tanking out of stance. TBs are the easiest damage to take in this game, its a pass fail mechanic, they usually have casts or tells, everyone knows about them, everyone is ready for them. Its in between those busters where people get complacent, and you need some form of mitigation there. TBN works horribly for this purpose, and its all we have.
Before TBN existed, did you see WHMs spamming stoneskins on the tank in between busters? Did you see SCHs spamming Adlos for AAs? No, you know why? Because shields aren't designed for that, heals and cooldowns are. TBN is not a cooldown, its a shield. We need a cooldown.