My guess (and you can bookmark this and laugh at me in a year if I'm wrong) is that we will all be laughing about the launch problems of this game a year from now and they'll have things ironed out. Others may not have the faith in SE, but I do.
This is exactly how I feel. A lot of people don't like SE's mmo team, but that's because they don't cater to everyone, they cater to a specific niche of the gaming industry, and their games are based mainly on their vision, not that of every random person that sends them feedback. A lot of the initial flood of players were destined to quit, I never expected otherwise. But as the game roots itself into the industry, and grows over time, it will continue attracting players of a certain niche for a long time.
Now sure, they do selectively take feedback into account, the upcoming update is proof of that. But they want to make a game and let you get used to it, rather than making the game according to what you're already used to. If that's not up your alley, it's understandable, and you're free to go play another game.
Personally I like their style, and I don't see myself playing any other mmo's. I tried a couple others, and I didn't like them. I found myself constantly thinking about how much better FFXI was, and ended up quitting after a couple months.
As of now, I have a linkshell with 60+ active players who are all enjoying the game a lot. Yes there are some issues that need to be ironed out, most of which have fixes on the way in that big update. None of them are that big of a deal though. And it could use some more content, which I'm 100% sure SE will implement in time. That's just how mmo's work. But it's already a better mmo than any other on the market as far as my personal taste goes, and it will keep getting better. I trust SE's mmo team to deliver, and from what I've seen so far, they have no intentions of letting me down.
But, long story short, and hypothetically speaking... If FFXIV failed, I'd probably quit gaming except for the occasional single player game. Unless of course a new mmo came around that I'd enjoy as much as I've enjoyed the FF mmo's. I just don't see any other mmo's out there that I want to play though.