Generally, when you complete the content of a video game, you generally move on to another game while waiting for new content to be released, which is totally fine. But that can be avoided by providing replay value to the content (like, making something fun enough to be worth playing a large number of time, for example by making the players adapt to changes in the pattern of the content), which is sorely lacking in FFXIV, as the battle system is too simple and lacks depths and synergies (you basically only DPS and heal, as a class, and have minimal amounts of reactive utility spells).
Also, it is true that old MMORPG have an history of trying to make players stick to the game by implementing grinds, which isn't good game design (and the reason why MMORPG were not popular before WoW, a game which reduced the grind compared to older MMORPG).