You're right in that anyone will get burnt out on any game (or anything) at some point, but depending on how that game is designed the time until burn out hits could be minutes (tic-tack-toe) or hundreds of hours (Skyrim). An MMORPG wants that burn out point to be as far away as possible, because it keeps subscribers and makes the developer money. MMO Players want the same thing because it gives us an enjoyable game for longer. The issue with FFXIV isn't mainly "Not enough content", but instead it's "Lack of variety in content". FFXIV releases robust patches at a good clip, but the content released also contains the same basic premise as every patch that came before it. To make matters worse, every patch replaces the content from the patch before instead of adding to it. These things make us hit the burn out point much faster than we should, thus our suggestions in this and other threads.
Mildly off topic, but did you ever play FFXI during the Treasures of Aht Urhgan expansion? They had a piece of content called Besieged where the various Beast Tribes would build up their forces and attack a hub city every so often. The players would fight them back alongside important npcs inside the walls of that city. Winning gave the entire server rewards and the individual players based on their participation. It was and still is the most epic thing I've ever done in an MMORPG.
In FFXIV it has been bothering me that the most important summons in Final Fantasy history have basically been monster-of-the-week for the 2.x patches and we haven't seen or even talked about them since (with one exception or two). I always thought it would be awesome if Besieged returned in FFXIV and the beast tribes attacked our hub cities with their primal leading the charge. This would give us new battle content as well as keeping some iconic pieces of Final Fantasy relevant beyond their brief appearances in the 2.x story.