Threat isn't the only viable tanking mechanic.
Eg. I can't remember if they implemented it or not but I remember the novel take on this that Champions Online said they were going to use was a debuff applied by tanks such that if the target attacks anyone but the tank, their damage is reduced.
That's functionally a different way to achieve the same goal of the tank, i.e. damage mitigation for their buddies.
Yeeeeeah, how does the mob decide who it is attacking though? If there arnt explicit aggro skills, its probably just going to be on account of who is hurting it more.
There's that delightful attitude we all associate with the damage meter crowd
Two miners come out of a mine, one has a clean face, the others face is dirty, the first one sees how dirty he is and washes his face, the other sees how clean the first is and remains a filthy casual forever. DPS meter is the mirror.