This was likely because the assumption was that players would play one or the other, not both. The problem is that the SCH doesn't really belong on ACN, because if you look at how the stats change with SCH, the SCH job stone brings the stats only up to the level of CNJ.
Everyone focuses on this, but incorrectly ignores that ALL the Jobs required leveling half of another class first. This is a throwback mechanic first seen in Wizardy a CRPG, that predates Final Fantasy and is in fact was more popular in Japan than in North America.
The correct way of fixing it would have been to simply use the FFI style jobs like they were originally designed (eg inspired from Wizardry) where you start as a class and you upgrade to a more powerful Job. Some of those jobs are a direct descendant of their class (eg White Wizard has White Mage.) Where as in Wizardy, there were 4 starting classes (Fighter, Mage, Cleric, Thief) that upgraded to Elite classes (Wizard (Cleric+Mage), Samurai (Mage+Fighter), Lord (Fighter+Cleric), Ninja (Can use all gear, but best ungeared, basically a Thief that can use all gear and no magic)) and this is what the cross-class skill mechanic was for.
Nah, if they move away from it, it's too much like "every MMORPG" and less like Final Fantasy. The jobs give you those extra skills and stats, they aren't just a continuation of the base class.
They are probably making the three new Jobs something more like "Elite Jobs" in the Wizardry sense, where you need to have at least one class at level 30 and simply having any class at level 30 unlocks them using new gear that starts at level 1 (30.) eg You enter the "Astrologian" guild in the V3 area and the NPC will go "I see you are a healer, would you like to join the ASTROLOGIAN guild?" , which would then unlock new gear that you as a SCH or WHM can equip. At that point at level 30+ would also unlock a quest somewhere else for the job stone.
For DRK, it seems pretty logical to follow the same rule, new gear that they can equip and a new level 30 stone quest somewhere else. For the DPS to unlock the machinist... seems more like one would just wander into some "machinist garage" as a DPS and have a conversation like "Hey, I see you like to break things too, want to learn how to fix stuff you break?"
All in all, Final Fantasy is strongly story-based. It would not be fun to simply be handed new content and not have to work for it. If the dev's say you can't create new characters in these new jobs, then there must be a storyline reason for it.