We don't need specialized jobs branching from the same base class when we already have the ability to level every job and class on one character. Specializations exist in other games because leveling all jobs on the same character is not an option.

It would be awkward at this point to try to shoehorn in explanations of why job X and job Y have class Z in common when each already has its own lore established. Your personal ideas appear to exist solely for leveling convenience without regard to lore, which is important to the game design even if it's not important to you.

As others have pointed out, the developers said on multiple occasions that SMN and SCH have been a balancing nightmare for them that they don't want to repeat. As it is, SCH doesn't even feel like ACN once you unlock it at 30 because what abilities are retained have so changed quite a bit from what they did as ACN. SMN has the same problem but to a much smaller degree. They may as well never have shared the same base class outside of the lore connection (both use arcane geometries for casting spells and summon entities to assist in battle).

You aren't required to level all 18 combat jobs currently in the game, nor will you be required to level whatever new jobs get added in future expansions. If you don't want to go to the effort of leveling a job, don't do it.