Also, I think I should add this. I helped lead an endgame LS for around a year. Sometimes we'd do sky, sometimes we'd do sea, sometimes we'd do HNM (sometimes we claimed them). We never felt any huge loss when we lost claim on an HNM or something, because our enjoyment of the game didn't depend on a certain piece of gear. Our ability to work cohesively as a group made far more difference than one paladin having a defending ring. Thus the content was completely optional. A similar level of satisfaction could be brought by force pop NMs. Personally, sky and sea were my all time favorites. This doesn't mean I didn't have a lot of fun, especially with lower level HNM like Roc or Simurgh. Sitting there 3 hours was not the most fun, as most of us agree, so we've proposed sufficient measures to fix this. Like OP said though, this doesn't mean we shouldn't keep a couple of long windows around.
The simple fact is that all content is optional. Even in XI, if an HNM dropped the best piece of gear I could have, I didn't feel like I was missing out just because I had to have AF2 instead of dalmatica (or whatever, I can't remember which was better). Similar yet different levels of "eliteness" could be achieved in XI whether you chose force pop, instance, or open world. I would like the same to be true of XIV. I would also like to address the grievances people have with XI's system in order to achieve the maximum level of support. But in the end, as everyone's said, the best way to do this is variety.
All content is OPTIONAL. If you feel like it's not, you need to take a deep breath and turn your computer off for a day.