That's incorrect. It is also the reason target lock is being added.
But honestly think about it, why 20%?
If any group is at 20% and can lose the mob they shouldn't be fighting that mob.
A group of level 80 to 100 players SHOULD be fighting Falgren, for example. It is their right to enjoy the game. It is their right to play out the quests and gain the armour that we all have had the chance to get. It will take them some time to kill, however.
Meanwhile three level 140 and above can slice through Falgren and get the kill even if the initial group has it down 20 or 30%. The high levels are doing it for profit or for their alts.
I am certainly guilty of killing Ducarius, Avatar, Kiara et al. for profit if there is no one fighting it.
In another instance, think about that Tempest cloud. It is level 30, and designed for level 30-50. Players in that range are correct for the fight. A level 100 could easily swoop in an ks them.
I challenge you to find me a group of level 50s who were able to kill the Crossroad Wyvern in the summer (certainly a boss for their level). In reality, level 100s and above camped the spawn and killed it in a few hits.
I agree with you if you are talking about a group that is fighting above their level. But the problem is that far too many people are fighting below their level... myself included.