I dont understand though, if you are a career tank, this is nothing new to you.

Paladins have no TP management as an OT. Never had.
Warriors had none what so ever, since you had defiance off, you never had any stacks to use as an off tank.

Warriors just in Heavensward got some TP management and 'things' to do with their stance, but the other tank is just as bad off as you are. So how are you not viable? Warriors are better OTs TP wise, and you are tied with Paladins in your TP efficiency. Unlike a Paladin however, you have a many more abilities that can damage that use no tp. If you ever went OUT of tp, you could use mana abilities to keep SOME damage going (only notable since a paladin with no TP is purely auto attacking).

I don't get the issue? You cant have your cake and eat it as well! DRK have incredible aoe, very good burst capability, better sustained healing than both tanks, mobility and other advantages.