Well, whatever the "objective" argument is, apparently there are many people who think like me since, unlike other series that (arguably) spun off from FF like Mana, SaGa, and Chrono Trigger, Bravely Default and Barvely Second are covered very extensively on the FF Wiki. Even if we do treat it as one of those IPs, it goes without saying that it is the one that has the most common elements wih FF out of all of them

Also, according to the wiki in question: "Although the game has an original title, it is considered a spiritual successor of Final Fantasy: The 4 Heroes of Light, a "gaiden" or "spin-off" of the main Final Fantasy series, by its creators. The game follows the traditional Final Fantasy storyline of crystals and the Warriors of Light with the traditional job system gameplay, and the "Flying Fairy" in the Japanese title alludes to Final Fantasy by having the same initials."

So the creators basically consider it a spin-off, much like Crystal Chronicles, and not a separate IP like Chrono Trigger or Mana.


And here's a list of FF allusions within the game, just to finish this little post off.