I believe there are a few main reasons for the FFXIV population decline. There is a not insignificant number of people (and growing) who have become exhausted with the job and battle mechanics, and general homogenization and lack of complexity or decision-making within the game combat. Folks looking for a game which requires more than pattern memorization are starting to look elsewhere.

There's also the argument for lack of casual and/or mid-core content. Also lack of any long form grindable content. SE has done a pretty good job with bringing the more difficult "party finder" content to the game, but that even now is only servicing a small portion of the playerbase. The larger majority currently has nothing to do, and hasn't pretty much since DT launch. They are steadily leaving the game due to lack of content. This has always been strange to me because FFXIV is a big enough game, and makes enough money to have a team capable of putting out content for all types of people without sacrificing what they are already doing. It doesn't have to be one or the other. Everyone can be served, but that's not happening.

On a personal note my sub is expiring tomorrow and I have no intention or renewing it. I may come back for 7.2, but we'll have to see what that patch actually provides. If there simply isn't enough to do I won't bother, and I know I'm not the only one who feels this way. This may turn out to be a "wait for 8.0" situation, but we'll see.