If this is the case, why did FFXVI not completely fail? After all, it had to tell a completely original tell, do all the setup for itself and build brand new characters. All while, coincidentally, has several of the FFXIV staff team and writers.
Could it be those writers were simply better and the person/team they assigned to Dawntrail was mediocre at best? By your line of logic, no beginning book in a trilogy could ever be good since it has to do all the legwork of setting up future sequels. Ironically, a lot of Shadowbringers works because Ishikawa made several retcons to fix some rather poor decisions previously expansions made. A prime example being both Lahabrea and Elidibus. Neither were remotely the characters they wound up being when first introduced but she was given free reign to alter them. Which speaks to their confidence in her writing talent because they already attempted that with Lyse to mixed reception.
Dawntrail didn't need setup to tell a good story, especially because it wasn't trying to be a continuation like Shadowbringers and Endwalker were. It needed a good writer to craft a cohesive narrative that was mostly resolved by the end. In other words, Dawntrail was more like a single player game since we don't necessarily know if they're planning to do another drawn out arc or stick to more self contained, shorter stories. Regardless, Dawntrail failed spectacularly in that objective. The pacing is completely horrendous even within its own structure, nothing of note happens for several hours worth of gameplay and virtually every character not named Wuk Lamat is rendered a hollow shell of themselves only there to prop up the clear "main character" who hogs the spotlight to the point of strangling it.
Shadowbringers would still be a good, if not an excellent story even without any previous setup whatsoever. Things would have to change to better accommodate that direction but the core aspect of its narrative are relatively self contained. Conversely, Dawntrail is a weak story made weaken by how little desire or respect it seemingly has for any previous character.