Thats the problem...if we trust Yoshidas words from Reynharts quote then no Eureka was not casual content, it was always meant to be a niche that only tickles the interest of specific players. Which is exactly what happens.
But if we do see Eureka as casual content (and honestly the size and amount of stuff done for it should hint towards it being meant as casual content, especially since the once casual relic is in it too and one of the ARR story question is solved there too) then its an utter failure in that part because even on the JP side only around 19% of the players reached Pyros level cap. So most of the players never went that far and I kinda doubt that the numbers will rise by a huge amount in the future.
Edit: And lets not forget that these 19% was only reached at the JP side. While the bigger playbase (EU and NA combined) did not even reach that together..
All of these numbers for content that was not only advertised quite some time before it was released, got even delayed further which made us start the relic quest later than normal, had the Krile story line in it and what happened to the one isle that vanished in ARR, had the relic quest line and relic gear in it, a couple of mounts, a big amount of minions, TT cards locked behind its progression, music scrolls, emotes, and other glamour..and yet especially on the NA side not even 10% have reached level cap in Pyros.