DRK has a variety of problems not just TBN. There is dark mind only being magical damage, there is abyssal drain which is very meh to say the least. There is Bloodweapon that should have charges. There is salted hearth and salted heath 2 that are just trash. His aoe rotation is all magic when it should be weapon skills. There is oblation being underwhelming. There is Living dead that will continue to be bad for another expansion, this is going to be the 6 year in a row that happens. There is Delirium being a bad copy of inner release.
So right now he is the one doing less damage for tanks but at the same time he has TBN, in 3 weeks he will continue to do less damage than the other tanks and his mitigation will be worse since everyone got better versions of TBN. So why would you even play this job?