They've said they're actively trying to avoid that because of Scholar / Summoner and all of the coding pains it causes, even though it's objectively a good system. Even if you needed to "cheat" under the hood it would be very nice for new jobs to be tied to old jobs instead of just being their own new thing. Berserker from Warrior, Geomancer from Monk or AST, Mystic Knight or Divine Knight from PLD, etc.
They don't have to share EXP (which I think is where most of the SCH/SMN pain comes from) or a class, but they could share most gear. Would alleviate some of the problems with group making at least.
This isn't too far off what the jobs system kinda used to be - you'd have base classes, and depending on which set of them you'd levelled, you could access jobs as upgrades. PLD used to require having Gladiator at 30 and Conjurer at 15, for instance. Dragoon required LNC+MRD. So on.
And I think the initial plan was probably that you'd have multiple jobs, like SMN/SCH off ACN, for each class. You can even imagine some pretty obvious ones here: THM+CNJ for Red Mage, PGL+GLD for SAM, etc.
But SMN/SCH splitting off ACN was a failed experiment because it was a balancing nightmare - the devs found out pretty fast that because they shared so many ACN skills, any adjustment to balance one side of things had major unintended effects on the other. So they dropped the classes for 3.0, and just started doing jobs that didn't have base classes, to avoid that headache in the future, and never did anything like it again.
Which I think is a shame, because it's a cool system and I think it could have been made to work, but it's never going to happen, not after a decade of doing it the other way. What I expect they'll do is just doubling down further on what they've been doing since Shadowbringers - sanding off all the corners and identity until jobs are just visual flavor options, all playing the same and with all the same buttons as the others in that role. Looking particularly at the healers here, but the tanks aren't too far behind, and DPS are catching up.
No need to worry about the fact that there's two dozen jobs to level when you can just level the one inside a role that you like the aesthetics and flavor of, and be safe in the knowledge that it plays the same and does literally everything that the others do.
