I disagree. It certainly wasn't a fault with FFXI, which is the other MMO I've primarily played.
The main issue facing FFXIV (and many other modern MMOs) isn't just a relative lack of new content - it's that the intensely vertical design that renders new content so necessary. As an example, a year and change into the ToAU expansion for FFXI, there was a vast reservoir of content available, because literally all of the stuff introduced in Zilart and CoP was still relevant. So if SE only added one new thing every six months, that was fine: burn-out was much less of an issue.
In FFXIV, virtually no content is relevant for more than six months. Rabanastre just turned six months: now, it's just potential filler if I queue for an Alliance Roulette. We have a much smaller collection of things to work with: usually one Trial, one 24-man Raid, one end-game Raid, one Ultimate fight, and 2-3 dungeons. That's rough, and it's a far cry from where FFXI was at this point in time, where you had Dynamis, {sky}, {sea}, Limbus, ENMs, Salvage, Einherjar, ZNMs... on top of loads of more middle-class content like Besieged, lesser NMs with nonetheless decent rewards, KSNMs, Assault, Nyzul Isle, and a host of money-making activities (since Gil was far more relevant in FFXI).
Amplifying this, on a personal note, is SE's habit of advertising all content for a patch cycle without distinguishing from the get-go when content will launch. I'm still salty over Heaven-on-High being a mid-cycle piece of content, something that wasn't mentioned whatsoever on the 4.3 Special Site, and to my knowledge wasn't officially announced until the preliminary patch notes.
I would, but as we well know (certainly as SE knows), MMOs appeal to people for reasons far beyond the game itself. Most MMOs, in some sense, are actually kind of terrible games: they don't typically feature the best plots (XIV and XI are exceptions here), they don't have the best graphics, they have server issues, they can be expensive, they typically don't feature the deepest gameplay mechanics. What they offer, however, is a world to run around in with friends, and an amazingly strong sense of allegiance to an alternate identity. I remain very attached to my character from FFXI, even years after I logged out for what's likely the last time. I can remember my Mog House, friends made, places of Vana'diel that were particularly beautiful or nice places to sit. These are not things that SE created, for the most part: they're things that naturally arise when players are put into the proper setting.
These aspects mean that FFXIV will continue to be something I play. I enjoy the time I can spend online with my girlfriend, particularly when I'm traveling, as I am now. I have an attachment to my house, and my character. I'm not someone who is currently ready to quit - though if my house were ever demolished somehow or my girlfriend to lose interest, I'd surely leave and never look back.
So, as long as I'm here, I very much want the content to be appealing. There are a number of ways to accomplish that, but so far, SE hasn't been very successful, which is frustrating. Broken systems languish for too long, like a lack of housing availability for quite some time, or the still-clunky Glamour system. New forms of content are poorly-designed, like Diadem or Eureka, and progression continues to be sharply vertical, reducing the appeal of dungeons, trials, and raids. Each patch I get my hopes up, and each time I am met with disappointment.
I'm not an innately pessimistic person, however, so each update cycle I experience until I do end up leaving, I'll hope the game shifts course. I'll also continue to voice my opinions on the forums, because I think it's valuable feedback for SE, and I know I'm not alone in feeling this way, and I want to encourage a feeling of solidarity among people who similarly want XIV to grow in new directions.
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I am curious, however: do you honestly feel as if the content additions we receive are sufficient? I mean, obviously if one plays only 5-10 hours a week, there's plenty to do - so let's separate out that argument, and examine, say, a piece of instanced content. How long do new 4-person or 24-man instances hold your active interest, or do they always hold it? For me, as an example, Rabanastre felt interesting for about 2-3 runs. After that, I didn't interact with it beyond when the Roulette landed me there, and it didn't feel in any way different than Labyrinth of the Ancients, which released years ago. As such, it didn't offer me much value. Is this not something you experience, or do you experience it only much later?