How are you going to deal with base classes as opposed to Jobs? In-game they're pretty much a direct upgrade over the base classes, but generally things are more complicated than that if you're writing the world as if it were real.
For example, the Paladin is a direct upgrade of the Gladiator, but in-universe the only real difference between them is that one is a knightly order and the other isn't. There's no reason a Gladiator couldn't become just as dangerous as a Paladin; potentially even using the same techniques, or something analogous to them.
Likewise, the first major difference between the Summoner, Scholar, and Arcanist, is their summons. And while I doubt anybody would argue that the Summoner egis are more powerful than a common carbuncle, or that a fairy has more utility, the rest of their magic doesn't seem to share that same distinction. It's just a matter of those advanced spells being lost and needing to be rediscovered. And in a living world I highly doubt that we've truly seen everything the Arcanist guild has to offer in the first place, meaning they may be studying or developing spells at that same level, just in a different direction, that we never get to see.
And it can't be that hard to make those high level techniques. After ARR, every job you get is something you can just jump into, rather than needing to upgrade from a class. Including the jobs that are completely new to the current era, like Red Mages or Mechanists, or jobs you have minimal support or supervision in like Astrologen and Samurai.
And I know Job classes cheat by storing memories of their past practitioners in the job crystals, but there's no reason the class guilds couldn't start stocking up on those to distribute to their most advanced members, either. Heck, the Mechanist people outright state the crystal is empty and you'll be mostly on your own for developing new techniques.
So, I guess here we get to an actual question. In this story, are the Eorzean class guilds really that far behind everyone else? In game terms, they'd be maxing out their potential at a mere level 30 or so?