A new player that purchases "A Realm Reborn News" along with the expansion needs to complete the 2.0 storyline prior to even entering the expansion zones. This means at a minimum (by my count) completing twelve 2.0 dungeon runs and ten 2.0 trials.
When the expansion comes out - can you imagine the queue times for some of these old 2.0 dungeons and trials? If I'm a new player and I have to wait through 15 to 20 hours of queues just to get caught up ontop of likely 100+ hours worth of outdated storyline quests / dungeon runs I think I'd probably give up.
The Zodiac questline give incentive for players to play old outdated content.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).