Are we sure that the Ivalice games aren't just a set of shared races, locations, and motifs? I mean, you can get Balthier in WoTL, right?
Positive. Even Balthier's appearance in WotL could be considered canon. He was after the Cache of Glabados at the end of Revenent Wings, which (interpreted through inferred dialogue) sent him into Ivalice's future, during the time of the War of the Lions.
Taken from the wiki concerning his recruitment in Tactics:
"After the battle, Balthier explains he is a sky pirate looking for the Cache of Glabados. Ramza thinks Balthier is referring to the auracite, and when he asks Balthier what the cache is Balthier only mentions it is the reason he is currently in Ivalice."
Even writing this encounter out of the picture entirely, it's 100% certain that all Ivalice games (with the possible exception of the conjured Ivalice from FFTA) are on the same calendar in the same world. Vagrant Story is the one outlier, which shares motifs and names with the rest of Ivalice but was only later retrofitted into the setting itself, which happened around the time of FFXII, IIRC.
Ivalice's wiki article
The dates all line up and the world all comes from Matsuno's mind.
I'm not so much saying it wasn't thought out as I am that it comes across as an alibi, even though it's just how things happened in development. When FFT was first made XII wasn't even an idea yet as a numbered FF, so the lore of non-humes that used to exist wasn't meant to be expanded into a full story of its own...yet. Then when XII was created, the vibrancy of Ivalice's golden age was overshadowed by the lore of FFT. Anyone discovering that would be accordingly bummed out, I think.
The creative process doesn't work in a linear fashion. From an out of universe perspective, yes, the Viera, Bangaa, etc. were thought up after Tactics for certain. However, they were made to fit into the pre-catalcysm world based on pre-established lore via the cataclysm. Everything ends up fitting together in the end.
Think about it. A group of devs sits around a table and sets out to design a distant prequel. They have previous lore to work with, but they also have to create a totally original world. Of course it's not going to have been planned out from day one, especially since Tactics was a spinoff and XII is a mainline entry sharing its world made ten or so odd years later.