The longer people stay, the far less likely they are to leave simply due to the human nature to not want to give up on something you've invested thousands of hours into. WoW's players are some of the most attached because they've been there since launch, so while subs may dwindle, they'll stay strong for years to come simply because people are reluctant to walk away from something they know and have put so much time into.
If WildStar or ArcheAge were more of a threat, SE would be in more trouble atm than they are, but when there's no competition, you can be mediocre, or even crappy and people will stay if only because there's no better option, but it doesnt mean FFXIV is a "good" or "great" game, just "least bad" of the options out there. It runs good, looks decent, but it relies on grind (thats not entertaining or fun), and RNG/Fate Camping far far too much.
Simply the fact the game has FF in the title will keep it alive and well, esp in Japan. Whether or not that will be enough in the US/EU to keep the subs up I dont know. I'm trying to hang on until 2.4 before I decide my long term MMO plans, but with some of the rather stupid things SE has done of late, they need to get this train back on track soon, before some other game does come along and pose a threat. You can only coast for so long after all.