Did anyone claim that everything was planned out? It's evident from the material ingame that there is a plan for the overall story of Hydaelyn/Zodiark/WoL/Ascians. They're doing it the same way the Trails saga was planned out: they have the overall story but they fill out the short term storylines as they get to them. There have been only a handful of retcons for the 2.0-5.0 story. Most of the retcons have been trying to reconcile the 1.0 lore with the current direction of the story.
- - - Updated - - -
One thing I really like about FFXIV's storytelling is that we are actually getting to the ending in a timely manner. The plot as advanced significantly with Shadowbringers, and it feels like 6.0 could be the ending. Contrast that with Trails, which has been in comparison more messy. SC didn't even have a proper ending, so a third Sky game had to be created just to give closure to SC. Then, rather than heading to the Empire, the saga then got derailed by two filler games (*cough* Crossbell *cough*) which turned out to be pointless in the grand scheme of things. Not only that, due to a lack of foresight, Crossbell couldn't get a proper ending, and since they had set up their fans to expect closure to it, a significant chunk of the Empire arc was bogged down in addressing Crossbell. Now, we are in a situation where, rather than heading to the next plot point, we are instead getting yet ANOTHER FILLER game that is meant to address the problems created by the filler arc. By Kondo's own admission, we still have at least THREE ARCS left before we reach the end of the saga, and that's not counting if we get even more filler games. At this rate, we'll be lucky if we reach the end of the Trails saga before 2030.
(On that note, it's very fortunate that Shadowbringers didn't turn out to be a filler expansion like WoD. MoP was clearly setting up Legion as the next expansion - the developers even confirm that was the plan at Blizzcon - but since Blizzard wanted to capitalize on the Warcraft movie hype, we got WoD, which managed to screw everything up while simultaneously being a pointless filler arc).
FFXIV has done a great job keeping expansion arcs contained. Sure, the Ul'dah coup was aborted in an unsatisfying matter, but we don't have any growing plot tumors bogging the story down. So far, the only expansion plotline that wasn't wrapped up in the expansion it began in is Zenos, who seems to be more relevant to the saga as a whole rather than that one storyline.
- - - Updated - - -
The only other long running game story I can think of is Guild Wars 2, which managed to have a satisfying ending but required a few changes to get there. GW2's story was about six dragons (which are more akin to incomprehensible, lovecraftian horrors) who were going to wake up and destroy the world. The main protagonists are an adventurer's guild that broke down after an attempt to kill one of those dragons, Kralkatorik, ended with one of their members dying. The story of GW2 is about going around, putting the band back together to kill the dragons, with the end goal of killing Kralkatorik. After the first expansion, the team realized that if they tried to give every dragon their own expansion, the main story about the guild would take too long, so the remaining dragons were quietly written out of the story (they just don't wake up) and the story marches onward towards resolving the end. We got a very satisfying ending to the GW2 saga last year. We're still getting new storylines, but the overarching mega plot is over.
- - - Updated - - -
Fortunately, whoever is managing the Ascian storyline didn't overscope it. It very easily could have turned into "you have to kill each of the 13 Ascians one by one", but Shadowbringers neatly avoided that by just having 3 head honcho Ascians who raise the others up, so we all we have left is to deal with Elidibus (no more Ascians after he is dead) and Zodiark, and possibly the Sound that caused the Terminus. We don't need an entire Garlemald expansion (though it'd be cool to visit it), or need to liberate every single province. We just need to get take out Zodiark and save the planet, and we will get there within the next few years. (god forbid if we had to go to every other shard, holy crap I'm so glad they're not going to drag this saga out)
Anyway, I am very satisfied with FFXIV's mega plot. A lot of people seem to think Stormblood was the worst thing ever, but at least 4.0 was short and we still got a lot of plot progression, and I found it very entertaining all the way through. The overall story has been really tight.