OHKO need to exist or players abuse the fkk out of the encounter, people will completely ignore the encounter mechanics as there is no punishment, the party requirements will be scuff, Most will opt for 1 tank 1 healer if there is no punishment for having said comp.
take a look at the latest Extreme, IMO is heavily undertune in terms of damage for tanks and overall mitigation requirements, so what do people do when they know they won't get punish by Tank busters/demanding mitigation, you just SOLO tank/heal and ignore the last 35% of the fight.