You've hones in on a lot of my own issues with the recent expansions.

Our villains have been too self contained. They do not extend beyond their immediate motivations. Now this does have a narrative purpose. It makes them easy to digest. You don't need to play prior expansions to understand the goals of THIS expansion's big bad. But What made ARR - StB so beloved is that all three expansions were tied together. We fought corruption and brought down tyrants, but there was still an overarching evil at play. Something to connect everything.

That changed in ShB. Now don't get me wrong, ShB was an amazing expansion. But it was the first nail in the coffin. Our overarching evil was personified with Emet-Selch. He was a wonderful character that brought everything we had been working for to the fore. An end to the Ascians and an end to the Calamities.

But we were separated from the core of our plot. We were not in Eorzea. We did not have the Garlean threat nipping at our heels. It was replaced with the very cool but self contained Sineaters.

Then we go back to Eorzea in EW but we aren't reading the same story anymore. We aren't going to stop Garlemald, we are going to save it. We spent one expansion away from this ever present threat and we disconnected from the terrible atrocities they committed. They became pitiable. And the people of Garlemald are pitiable. But the armies and legions are still out there. But everything is hand waved away. Neither Garlemald nor Zodiark, the greatest threats we have been training to defeat, are just mid game fodder. Suddenly there is a NEW terrible threat that's going to destroy not just the star but the whole universe!

All this to say that the expansions have become stale because they are self contained. They don't connect to a greater narrative anymore. And because of that, our villains feel weak and watery in comparison to the villains of the earlier expansions.