Eureka... what were you expecting?

[Fyce]

6-8 people around your level. Which may be the problem later. We're going to have 30-40 lvl 20's farming their stuff and a scattering of 1-15's who can't party with each other and have to grind solo. This is the problem I foresee. I have no difficulty completing NM's around my level in a party, but if I'm playing catch-up it's not going to be like that. I really hope it's NOT going to be like that but I don't see a solution currently in play or possible without changes.

later subscribers and players are going to have a much worse experience than those currently playing.

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.

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.

I'm actually very surprised that SE managed to come up with a content which seems that it'd hold abandonment pretty well. And in regards to this thread's topic, I have to say that wasn't something I was expecting.
Time might prove me wrong, but so far it doesn't seems an actual concern to have.