[Theory] ACN removed, SMN remade

[Krr]

And what do you do about the many arcanists?

Give them Summoner and Scholar levels are their current Arcanist level.

What of other classes? The entire early quest line would have to be scrapped and redesigned to fit the new job-only system,

No they wouldn't. You re-tag every lancer guild quest as a dragoon quest. You re-tag every conjurer guild quest as a white mage quest. Your character's lore story remains the same, just minus unnecessary and confusing legacy mechanics.

and lore-wise people would still have job crystals so what happens if they unequip them?

Nothing. Your job stats and abilities are folded into your base class. I mean really, all that oh-so-important crystal does is give you some bonus stats, 5 abilities that you want all the time anyway, and force you to rearrange your entire skillbar when it gets reset back to the hideous default at level 30.

Lore-wise you're the only living White Mage left in all of Eorzea. Why hasn't S-E deleted all the others from the game yet?

There's far too many variables for them to give it such a severe revamp, so it's unlikely they'll be doing away with classes entirely.

Except...no one has described any mechanical reason why we can't just retag every class quest as a job quest and give everyone job icons at level 1, and then say each job can take a limited set of abilities from two subjobs.

I highly doubt we'll see a Summoner remake, though.