Although we could have had another Garlean expansion, we had Stormblood which saw Othard liberated and the entire Ivalice and Bozja plot which sees the territories next to Othard liberated. It's not like we had nothing.
I doubt any expansion could pull off a war as well as Bozja did. Critical Engagements blow Stormblood out of the water in making it feel like a war/battlefield with trenches. It took until the very end of Stormblood just to give us the closest thing to that, the Ghimlyt Dark.
The Ascians were a pretty tired idea by this point. They could have milked them more and made a "Revenge of the Ascians" expansion, but the risk with that is letting it become a really tired idea. Sometimes ending a plot on a high is better.
If you actually ranked up and did all of the beast tribe stories, their stories were finished already. It only seemed rushed in the MSQ for people who didn't bother with them.
I don't know what stories could remain with the grand companies. They are politicians. All they can do is tell us about Kan-E-Senna's political priorities.
And Meracydia and additional information about what the allagans got up to.and yet now we stand staring at probably the last remaining thread from ARR, the voidsent.
New lore is being created already. The New World's mention in EW, in the blue mage quests and its visibility from the moon for example.But after all the streamer reactions have been siphoned, and the heat dies down, what is left exactly? A villain of the week scenario where we kill a generic big bad every expansion?
We already get a new "generic big bad" every expansion. Zenos was a new character in Stormblood, Emet-Selch was a new character just before Shadowbringers, Meteion was a new character in EW. If this continues it doesn't seem like it will be a problem because these were all great characters with lots of fans.