This is what is going to happen. First, a bunch of new MMOs will be released, and they will "fail" for the same reasons all MMOs flounder in the beggining. They are almost always, and necessarily, incomplete at release and then continually patched. Diablo III, is an exception, mostly because Blizzard holds back their games to ensure a quality product upon release. Worth noting they didn't do this for WoW because it is an MMO. I'm sure many people will play Diablo III, I might play it and FFXIV at the same time. But they're not true competitors, so I don't see how DIII could spell the doom for FFXIV.

Ultimately what will end FFXIV is FFXIV itself. If it is a quality product, people will buy it. If it is not, people will not buy it. No amount of negativity or optimism will change the actual product, save perhaps encouraging the dev team, so it is only in comparison of other games and their acceptance by the players that FFXIV would in any way be affected by these releases.

I'm thinking once PS3 release is set, FFXIV will look much more appealing to many of the newer MMOs at the time, and will have a lot of actual players to help hype it.