The Zodiac questline give incentive for players to play old outdated content. That incentive needs to exist even after 3.0 is released. There needs to be some perk to having a Zeta that is long lasting or simply the Zodiac questline needs to continue (maybe side by side with a separate relic questline).
Oh. Don't worry, they'll find a way to make us play that "old content" again. With the three new lvl30 jobs first, then probably with another relic, or even crafting-airship-materials... or whatever.