People will leave and not come back.
I really don't see that happening. Actually I believe that the retention rate of customers who stayed in the last months will be close to 100%. People will -at least- give a shot at the beta (in late stage, when the game is almost in a release state) or at release timing. It doesn't mean they will stay after experiencing ARR off course.

But I just don't see how someone who endured XIV 1.x for 2 years would NOT at least give a shot at ARR. It doesn't even matter if you are playing something else, chances for you to come back and at least give it a shot are extremely high. If your interest is not high enough, you would have stopped earlier.

ARR could be released in 2014, I would still give it a shot. Even if it means playing something else for a year in between. And I am sure that lot of people would do the same. I can completely see SE making a "Welcome Back Campaign" when or soon after ARR is released, so former XIV players can get like 7 days free to "witness the changes".