This is nothing unique.
They could have had a special zone in the overworld that has the same effect. I mean, they're working on adding more cross-server support in ShB. Having a cross-server zone would have been a perfect idea to lead up to that.
So far.
There's literally nothing stopping them from adding in a max level zone specifically designed for max level players to do max level content. Outside them being less able to stick arbitrary ilevel syncs onto the content to bloat out the content so that it's more annoying to do alone or in smaller groups.
Shocking that when overworld is not designed with any overarching reward for playing in, that no-one is found playing in it.
Again, if Eureka rewards were tied behind overworld content. People would be playing the overworld content just the same. Farming FATEs just as they do in Eureka.
Again, nothing that couldn't be adjusted in a specific open world zone.
Also, I beg to differ about "More experience killing one monster 4 levels higher than killing 5 weaker monsters". My experience is that killing monsters at your level, thus will be the minimum level required for Chain bonus > killing monsters higher level.
Experience gains from increasing levels doesn't really scale very well with how much longer they take to kill.
To say nothing about how party level differences can also reduce experience gains, which Eureka has a lot of because even though everyone is level 70, they decided to put in an independent leveling system because...?
Like, the only reason I can see for independent leveling system is to punish people when they die a lot more. As well as make it harder for people to participate in content when they first enter a particular zone. Even more so when even more arbitrary gates are put into place like being unable to use a mount or attune to aetherytes before specific levels.
What you're saying is that the content just isn't good enough or actually liked enough to stand on its own merits.
That it needs such a highly valued reward to actually entice people into playing it.
Since, the argument you're making is that Eureka is supposed to be content for those that don't like Raids/Dungeons to be able to do something. Surely, the fact that Eureka is a way to get full ilevel 390 gear, just like Raids/Dungeons would mean that all these people who don't like Raids/Dungeons can have end-game content where they can farm up gear just like Raiders/Dungeoneers? (You know, outside the fact that Eureka throws TONS of Tomestones at you for doing FATEs so you can easily buy Tomestone gear...)
Or is it just the fact that Eureka makes gear worthless because ilevel syncs that would make people not care to grind it out for a set of plain old ilevel 390 gear?
So they require specifically Relic weapons to make the content appealing enough to stick with? If that's the case then it means Eureka just isn't worthwhile content to make if people don't like to do it.