Fun is after all a transitory feeling. What is a barrel full of monkeys today can be a barrel full of crap 2 weeks from now... so you're right, the content will have to latch onto us not only today but keep providing a challenge or a deep sense of reward for accomplishments years from now.
But isn't that the crux of it? Everyone's different and have different adapting and breaking points. Ideally, you want everyone, even the most casual starter person, to go in, get hooked, and then ease themselves into the more difficult content to stay for the long term.
But where do you draw those lines? I don't think even the best game designers in the world have the answer to that, because of how varied everyone is opinion wise. So, many appeal to the lower common denominators at the start without alienating your core target audiences, and then you keep tweaking your difficulty curve until you find the niche you're comfortable with.
Problem is, most MMO's can't afford that adjustment period. SE, being in a similar business standpoint as Blizzard, can. I'm more than happy to give them that leeway. I'll be honest. I'm having fun. Not only that, but they also have me invested. Why? Because I like Final fantasy, I like the lore, I like the tropes, I like the atmosphere. I'm willing to work with them, especially after seeing what I have so far, to give them that feedback from my own personal perspective. (That they will take with Dalamud-Size chunk-o-salt, but that's to be expected). I'm not going to bother to try to make a statement for the majority - I feel that's rather presumptive of anyone who does, especially if they think it coincides exactly with their views.
Everyone has left or has stuck with a modern MMO for their own reasons, because, honestly, everyone has different thresholds of tolerance, different tastes and ease of enjoyment, and different personal triggers for investment. Everything else is speculation or agreeing/disagreeing with someone else's perspective.