There's nothing new about players demanding fresh content. It's impossible to say Eureka or any fresh content is not a direct response to that demand; or rather, this is a given. It was purely the devs decision to place the relic inside Eureka. Player's didn't ask for that. From what I recall, players were adamant about not doing another FATE grind, and having to deal with RNG when progressing relic steps, and I think players ended up dealing with both inside Eureka anyway. Most players don't mind grinding out tomes to progress relic phases because they can accomplish this while doing the roulettes they are doing everyday anyway.
Since I didn't participate in Eureka, I cannot compare its grind with any previous or current one. Since the EX trial came along with the relic quest, we cannot say that the trial is a response to the relic being too easy. I'm not sure where you are getting at there, or I am misunderstanding you. The dyeable AF gear is in there which is I feel is fine as the dyeable versions of raid gear is likewise locked behind Savage clears.
It's important to give both new and old content incentive. Many of us keep demanding that content should continue to challenge and engage us, and healers in particular are demanding that encounter design be changed. SE is over in their area, looking at the low% of players participating in EX and Savage and is likely thinking, "Why? It's already there." I think they want to get this percentage up before really changing things up when it comes to encounters. There is already tons of rewards behind EX clears, but that apparently hasn't been enough. We'll see what happens with the AF being in there.
I never said Eureka wasn't successful. From the outside looking in along with a lot of opinions on the content being bad, bad, bad; I still get the impression that Eureka was very successful, and actually sounded like a lot of players were having fun in there. I think it had a rocky start but eventually got to that point of self-sustainability that it no longer needed the incentive provided by the relic to keep it in flight. I think going forward, we are going to get elements from both Eureka and tome grinding (visiting old content) to progress the relic. I don't need a crystal ball for that though; the devs stated that's what they want to do this round. The only question is how they will accomplish this.