We'll probably be back in Ul'dah and such in the expansion to clear our name and help those left behind (sooner rather than later). I think in this way they'll try to keep old zones and content semi-relevant so we don't just spend every minute in Ishgard. Otherwise new players might think "wow this game is empty" since they'll be stuck in old zones and won't be able to see a lot of the new stuff at first. That's just my guess, though.