Eureka... what were you expecting?

[Krotoan]

Wait, why wouldn't the scattered players group up? Fate EXP isn't affected by party composition at all. And since even a Lv.1 can be a valuable party member by simply being a healer/rez mage/SMN, there's no reason not to take them with you. The Lv.15 will have a really easy time making lower level Fates spawn while still getting a chunk of EXP and crystals, and the Lv.1 would quickly get fate EXP while providing support. (Granted, tanks and other DPS won't be able to do much though...)
Besides, the Lv.20 players would likely join the fates on the occasion, as there's not really anything to "farm" in Eureka at high level. Once you kill Pazuzu (Lv.20 NM), you get your feathers and... well that's it. If you want crystals you'll have to do fates anyway.

There are already high levels who come in, kill maybe 3 different NMS including pazuzu then leave. This is a thing. They don't want piddling low level NMS or anything that doesn't get them optimal crystals and lockboxes.
A 15 solo would be better off leeching off the high levels, the low levels would likely rather get XP as they try to spawn things. If they band together it's to HOPE that they can do better than alone.

The only issue being that lower level players will have a hard time not getting one-shotted by AoEs, but that why being able to choose what boss you want to fight is an invaluable improvement over regular Fate farm, Hunts and Diadem. The amount of control you have in Eureka is very high in that regards.

So, speaking out of my personal experience, I legitimately don't see the declining participation in Eureka to be much of an issue. Except in the absolute worst case scenarios... but even then, these probably won't occur before the content gets nerfed/improved.

A party wipe post level 6 will be DEVASTATING. All you'd have left at that point is HOPING that some level 20 will peel off and come help. otherwise hours of work would be gone. This is not how I want to play this game. I will concede others may like it, but I will vote no every time to this kind of mechanic.