Honestly if SE doesn't do something good with Zeta, a lot of people are going to be pissed off. It won't be the downfall of the game or anything but personally I'll be a lot more skeptical when it comes to any grindy content in this game and will participate less. When the Zodiac questline was announced they made it seem like it would be something that would continue forever, not something that would only be relavent up until the 1st expansion. People put literally hundreds of hours into those weapons and 5-10+ million gil and it would be a slap in the face to not at least give them some purpose in 3.0.

As for old content, you can pretty much guarentee that the new relics (or whatever they call them) will have you jumping through old, level synced content in order to ensure new players can get things done in a reasonable time. If you think you'll never have to do the old content again after 3.0 I think you'll be sadly mistaken. I could be wrong but I know they love to make you repeat all of the content until your eyes bleed so I doubt it.