Good lord Rokien, you strike again lol. First of all:
1) FFXIV =/= 2.0. I wish I could stop repeating this but alas, there are still those who (derp) dont get it. How can we answer this question when we haven't even seen the full game. At most we've played with a few of the mechanics and a few of the dungeons that will be in 2.0. You are analyzing a scrap product, that *will* be scrapped shortly. At most this is a 2.0 preview, like a demo.
2) To answer your question anyway (why i have no idea), longevity is a function of how much ppl want to continue playing the game. WoW is still active to this day and all it does is add raid, add gear for raid, repeat. You dont really have to do anything except create a world that ppl love. After that, give them new content every now and then and they'll be happy.
FFXIV 2.0 will do what every game does, add new content periodically.