From personally creating stories pretty much all my life, I can tell you its pretty hard to write a story on that scale set in stone. Unexpected plot changes can happen, or new ideas that might go in a completely different direction can happen.

Take JK Rowling's Harry Potter for example. She had a beginning and ending in mind, but the inbetween parts changed a lot from what she originally had planned.

Also take Rumiko Takahashi, a popular manga artist. She has stated before that she does not always know the outcome of her stories. She pretty much makes it up as she goes along. The result is that her stories are classics and have enduring positive remarks from fans from around the world. In her latest series "Rinne", she was asked if she planned to have the two main characters hook up, to which she replied something along the lines of "I'd really like it to happen! But we have to see how the characters end up feeling towards each other". As if she didn't control her characters.

I'd love to see the same thing happen for XIV.

Also I'm not really sure where you get the "starting anew" with expansion packs. Admittedly, the only other MMO I've really played hardcore was XI, but the expansions there built upon the existing storylines, while at the same time creating new smaller storylines.