FFXIV will succeed or fail on its own merits.
There will always be an audience for any MMORPG out there. As long as it's large enough of an audience to make the game worthwhile for SE to continue running, FFXIV will be around.
There's a sort of misconception a lot of people have that because "big title MMO A" is being released, it's a direct threat to other MMORPGs. I guarantee you the same, or similar, conversation is happening in the forums for myriad other MMOs currently out. "Will the release of "Big Title MMO A" cause "Existing MMO Title B" to lose a lot of players?". It's a recurring theme in every MMO's forums, every time a new one of any significance is being released.
The misconception is that if people aren't playing XIV, they're automatically going to be playing "Big Title MMO A". Or vice-versa.
In reality, not everyone is even interested in "Big Title MMO A" to begin with, whether they're interested in XIV or not. They might not be playing XIV *or* "Big Title MMO A". They might be playing something else entirely.
FFXIV will gain its own playerbase through its own merits, because it provides something to those players that they can't find in any other MMO... be it the type of setting, the FF theme, the battle mechanics... or most likely any combination of many things the game offers.
For example, the only other MMOs I'm at all interested in coming down the road are Secret World and, to a lesser degree than I had before, TERA. Secret world is attractive to me because I love the modern-world setting where "everything's true" in terms of conspiracy theories, mythical creatures, etc; also because I'm a huge fan of Ragnar Tornquist. TERA appeals to me because it feels like a spiritual successor to Lineage 2, a MMO I played and enjoyed for years; it was a wonderful PvP-centric counterpart to FFXI's PvE-centric gameplay.
That said, my intention is - assuming FFXIV works out well, and either of those two MMOs are as appealing to me on release as they are in development - to be playing FFXIV for one type of experience, and either of those two for another.
Whatever the outcome, I have zero interest in other big titles, like ToR. It honestly doesn't appeal to me at all.
I'm concerned about FFXIV's longevity based entirely on SE's ability to turn it around and make it a worthwhile title to enough people looking for this particular type of MMO experience. That's the trick. I have no concern that any one game is going to 'kill FFXIV' - unless it's a MMO that's *very* similar to XIV, trying to target the same demographic and does a far superior job of it.