I feel like that's largely the disconnect here? The expansion is adding more of the basic parts of it. The next steps of the story, the next series of leveling content, etc. Most of my curiosity for an expansion is in what ways is it adding to the game as a whole to change or alter something you do with the time in the game. So I do sit here and acknowledge it's adding a hefty amount of story and bringing more world to play in and more content to experience. But how much of the game itself is also growing so much as the list of things in the game growing (e.g., dungeons 50-57, primal 16-18, raid 3, etc.).
If this was a single-player game, I think I'd be more inclined with your viewpoint? What's this DLC adding to the game to experience, where is it extending the game. For me with MMOs, it's more about how is the game evolving or growing as a whole rather than attaching a new shade of experience. Stripping out the FF or MMO of it, if all I'm really chasing is the list of new content to experience, I'd consume it and move on to a new game and chase those experiences? I stay in an MMO because of the whole package and I'd like to see how the game grows larger / wider rather than just longer.
I think FFXIV aims to be, basically, a single-player FF game that also happens to have an online component. It's very story-focused in a way many MMOs aren't, and bends over backwards to make most content playable without ever having to talk to another person outside of hitting "Commence" in Duty Finder. I think that's kind of why expansions are done the way they are - the story is the new content. They expect you to be invested in that story, and to play the expansion for the story, even if you do not care about endgame raiding or the crafting economy or the gear treadmill. You want it because it's the new Final Fantasy story.
I think it's fair to criticize them for not doing enough to keep you invested in the game once you're done with the story, though (at least until the next MSQ patch hits). God knows I unsubbed from 3.1 till 3.4 or so because I just didn't give a fuck about endgame and there was nothing else to do but run EXDR every day for tomes. I don't think they're totally blind to this, though - Yoshi even talked about how part of the reason they felt they could forgo making one dungeon every other patch is because they felt there will be other ways people can get tomes now, and they can use the resources to try making new types of content that don't currently exist.
We talk about how this expansion isn't especially "expanding" the game, just adding more of existing types of content, but it's definitely not as if they haven't been adding new types of content over the years. Hunts expanded the game. Gold Saucer expanded the game. Palace of the Dead expanded the game. Aquapolis expanded the game. Diadem at least tried to expand the game, even if it sucks. It's not as if they aren't trying to make new kinds of ways to play the game, they just aren't putting it in 4.0 proper - because the point of 4.0 is to be a new story campaign. That is what they are prioritizing for the expansion, because they think that's the appeal of FFXIV.