I think the problem could be resolved through loot.
I guess in XI SE intended for a group to go camp Fafnir for a bit and than move on. However, it never happened because of the crappy drop rates.
If they added lets say 20 major HNMs to begin with, put a reasonable repop on them, and gave them a decent drop rate. Maybe it would work. A decent amount of NMs would spread out groups and if they got their stuff fairly fast they could move on. Fights should be pretty tough, but take no longer than 30-40 min. Rage timers included.
SE would have to avoid giving them gear like in XI though. The gear that was hands down best around for many years.
I strongly believe that best gear should be earned by hard work, not drop. More like a relic quest, but something players can chip away at everyday. Reward effort, not killing 1 mob in this case.

Another idea is quests where you would have to kill HNMs. It's could be a bit of a twist on abjurations.
You get a cursed piece of gear made with some rare mats (throw crafters/gatherers a bone) go to a NPC and he says go kill Fafnir, Behemoth, and Ada to lift the curse.
This way they dangle a carrot which keeps everyone around occupied.