Healers cannot generate infinite health. DPS cannot generate infinite damage. Why should tanks be able to keep threat from anything and everything? Managing threat as a damage dealer or healer has been an MMO staple since the time of MUDs. Because WoW and many of its clones did so doesn't mean its the way to go.

Honestly, a deterministic threat system in general is kind of boring and remove immersion IMO, and its a cop-out. All these years of research on AI algorithm, and we have fully deterministic boring encounters that players fully control. Even GW2 had it better, where each boss had different AI for how they did threat (some bosses went after high defense, some after low hp, some against range, some against melee...you had to replay the encounter a bunch to figure it out, and even then there was a few random elements throw in to keep you on your toes).