WoW goes through phases in which everyone resubs as a new patch with new content comes out, and then in the following months people slowly drop off and unsub or just stop logging in, but then resub and start logging in again as another new patch with new content comes out. In and out like the tide. This is what XIV will be if its successful.

If its not successful, then everyone will just stay gone and the game will die. SWTOR, and many other flavor of the year MMOs, kept the majority of their start population until about 6-ish months down the line, at their first tier-progression, and then the games suddenly crashed population-wise.

The real test of if this game will be 2.2 and 2.3 and if they revitalize the game or if the game starts hemorrhaging. I know a LOT of people who are subbed, but don't log in, and are basically waiting to see if 2.2 gives them a reason to log in again or if not: a reason to unsub.