Job identities aren't set in stone. DRK was initially described as a sort of parry tank. I suppose in fairness, we did have Dark Dance, as well as a number of abilities (Reprisal, Low Blow) which interacted with Parry procs. But you know what? In 4.x, there is not one single mention of the word "Parry" in DRK's job specific actions text. The closest thing that we have to a "Parry" tank presently is WAR, who has that odd bonus to Parry built into Defiance, as well as the only 100% Parry move in the game.
There is no design justification for lifesteal on WAR other than the fact that "we threw on some lifesteal abilities in ARR for the sake of it and never properly re-evaluated the job since." I'm not saying that this game shouldn't have a lifesteal tank. If anything, history has shown that it's an extremely popular and fun concept. But if WAR really, truly is meant to do this, it should give up all the other thematic concepts it is hogging. Why is it also the most mobile tank? Why does it have the best burst? Why does it have the best party-wide barrier shields? Narrow the focus a bit.
Meanwhile, SE seems to struggle immensely with what to do with DRK. Should it be a parry tank? A magic tank? A bubble tank? (Psst... DRK has a 27 year history behind it to draw on and a lot of in-game lore.) This game needs a proper lifesteal tank. That's something I'd like to see DRK properly commit to. We don't need the best burst. We don't need the best sustained dps. We don't need to have the best utility. We just need a core concept that people can get behind. And I think lifesteal is it. It would even make Living Dead's design more palatable: let us heal our way out of it.
With regards to balance: I presume you're asking if the balance has to swing the other way. If you could math out the perfect balance without testing, the answer would be no. The problem is, as we've seen with most of these theorycrafted "studies", is that reality doesn't match up with theorycraft. Even when you're the person designing the game. But I would much rather see the devs err on the side of change, rather than being overly conservative and seeing another two years of mandatory WAR/SCH/BRD/DRG/NIN.