In regards to Garlemald:
My thinking is, the Empire storyline and the Ascian storyline have diverged. While the Ascians are responsible for literally everything wrong with the world in XIV, including constructing the Empire in its entirety, 99% of the Empire is unaware of the Ascians' influence, particularly during Stormblood. The warmachine of the Empire has become an engine of destruction in its own right, which continues with infighting, conquering outlying nations, and the creation of Weapons even after the Ascians have fled the stage.
Shadowbringers was the point in the story that said "The Ascians no longer have a part in the Empire. The Empire is independent. And the Empire is still horrible." Even if we kill Elidibus, the Empire will exist as a separate violent entity until we go to Garlemald; even if we nuked Garlemald from orbit tomorrow, we would still have to deal with Elidibus and his remaining Ascian lackeys separately who might take advantage of such a Calamity. Dealing with one doesn't necessarily mean simultaneously ending the other threat - and in classic Final Fantasy fashion, finally dealing with one would really only make the other worse.
Now remember, this is an MMO: until the devs say "we're retiring the game, this is the final expansion/patch/whatever," there needs to be an engine of conflict to keep players not only coming back, but pushing into new regions.
The Ascians are on their last legs. We already have a hint of what is to come when Zodiark is gone. The only contribution the Ascians have left now is finishing them off completely. They are not that engine.
Considering Garlemald has been the driving force of the players' involvement for most of the story and the impetus of many of our pushes into new locations (including the entirety of Stormblood, the Black Rose threat sending us to the First, even helping motivate Thordan's counter-militarization), it seems unlikely the devs would remove Garlemald from the stage until they were prepared to replace it with an entirely new threat to serve as the driving force for the rest of XIV's lifetime.
For the WoW Refugees, Garlemald is like the Sargeras and the Burning Legion. This omnicidal conquering force that's constantly on the horizon, the threat of which served to motivate the Old Horde, the Illidari, the Scourge, and several player faction leaders; Sargeras has been a driving force of conflict for the entire Warcraft story. "Legion" was known as the "in case of emergency, break glass" expansion, because it was finally tying up many of the story threads that had been built for decades.
Unfortunately, breaking that glass also led to several hasty pushes to set up new enemies after the Burning Legion was gone, turning each expansion into a case of dealing with standalone threats as soon as they pop up, which is why WoW's player count has since dropped like a stone. Now that players have explored the entire known world of Azeroth, the only places to go are other worlds (where we can't even chase the threats we were invested in stopping when we started the game), or revamps on old content.
By contrast, Hydaelyn still has a lot to offer on its own surface.
In a practical sense, Garlemald would have to either be the final expansion, or be dealt with to clean the slate for a new power to take its place on the board. In order for that new power to take over in a satisfying matter, where the player doesn't just roll their eyes and go "great, we're playing whack-a-mole now," there needs to be satisfying setup well in advance -- something Final Fantasy has done very well for decades. Kefka killed Emperor Gestahl to set up the World of Ruin, Sephiroth killed President Shinra to set up Jenova, Ardyn cannibalized Niflheim to begin the Long Night, and so on.
(If you follow that trend, we have a potential setup being started with Zenos, but what makes Zenos a bad choice for this on his own is that he's a very short-sighted villain with no interest in conquering new nations or ruling from afar. Even if he attained godhood like Kefka, he wouldn't be interested in it to rule the cosmos, he would just want something to fight.)
But most importantly, the criteria for this hypothetical post-Garlemald power needs to include something that would push players into other regions of the world. Why would we later want to go to Thavnair or the New World, for instance, without the Empire as something to chase or push us there?