This seems to be an often misquoted or misunderstood fact that it was "originally intended to be 2 before being cut to 1 later" when in the original interview Yoshi-P said "1-2". What everyone takes as "oh, there were going to be two expansions and the first was just going to be Garlemald" was the interviewer asking what it would have been like if there were 2 instead after Yoshi-P's 1-2 remark.
They had to come up with the story in the Autumn of 2019 right after Shadowbringers released so that the cinematics team could begin work on the trailer, so all of this was thought up before the towers came into the game. Nothing was supposed to be one way or the other, just one or two and they settled on one instead of two and the story stayed the same but zoomed through at a breakneck pace.Originally Posted by Famitsu via Reddit translation
This brings me back to my point before of Shadowbringers kind of ruining the direction of the plot despite being critically well received. Stormblood keeps pushing the story into a showdown with Garlemald but then 4.4 and 4.5 start taking the Scions away to the First and the entire plot gets derailed by a side-adventure to rescue them from a whole other dimension and prevent another Calamity while "Meanwhile, in Garlemald" the war kind of just stops and stays in a status quo while waiting for the WoL to get back and NPCs end up doing things and going on adventures we should have done.
Either Yoshi-P, Oda, or Ishikawa said something in an interview ahead of Shadowbringers' release along the lines that the players may end up feeling they "revealed too much information" and they're right. The first 90% of 5.0 doesn't exactly move the game's plot forward as a whole, but Tempest and 5.1-5.3 let the whole cat out of the bag and revealed most of the backstory of the game. This and the overwhelmingly positive response to ShB seemingly forced their hand to wrap it all up sooner rather than later.
Two expansions after Shadowbringers would have let things sink in a little longer, but considering Oda said that the story would've been the same, the people in this thread would have had the exact same criticisms of Garlemald being destroyed off-screen and the issues with Venat.