As it is called expansion, I suppose the previous content does not dissapear. New players, maybe with some changes, will start with level 1 classes with the existing quests, with new ones but not eliminating the old ones. Probably the relic quest will still exist, and all the upgrades. Why make playable content disappear?
Nowadays the relic quest can be done in a couple of days (compared with months one year ago), and the Zenith upgrade in a few days. The Atma, Animus and Novus still take time and work, but less than they did. Atmas drop is higher but still takes time. The books are much easier to buy but you still have to fulfil them. And Novus still needs the materia and Alexandrites. Nexus is still the last upgrade so it is still a very long grind.
Probably these last stages will be made easier somehow: atma and alexandrite drops, and nexus lights are easy to change, materia prices can go down maybe with retainers bringing a lot of materia, books made easier, etc. I don't think it is likely they will disappear completely, it is still content to do by new players, probably they just will make it easier so they don't take as many months as the original players needed and new players can eventually catch up.
I suppose the relics won't disappear and either will continue to get upgrades or will be substituted by some other quest that does not start from scratch but from the last relic step.
The image of the weapon I think just means there will be a relic upgrade after the Nexus, since it looks like a Curtana but is not the Curtana Nexus nor any previous one.
