Most of the stuff listed in the OP doesn't even count as "content." Leveling a chocobo is not content, it's just another boring grind, the fact that there's going to be future GC ranks is moot because there's nothing you can do about it now, leveling other classes is not content, especially if you don't have an interest in said classes. The only actual content mentioned is PvP, which again is not everyones cup of tea, so telling someone they haven't done everything if they aren't a PvP player is also a moot point.

Anyway it's not so much a lack of content that's the problem it's the poor design of the game itself. Combat gets boring fast, there's no real difficulty, and nothing in the game lasts for more than 6 months so it mostly boils down to "what's the point?"