The way I understood it was that the new jobs are simply more focused play styles of the original classes. Monk being a specialty that you can earn as a pugilist rather than being a completely new job to level from 1-50(for example). That's how it came off to me in the battle reform blueprint anyway:

Rather than being advanced versions of classes, we would like players to think of jobs as specialized playing styles optimized for partying. While playing as a particular job will render you unable to equip certain actions of other classes, you will be compensated with access to exclusive actions, which can be acquired through quests.

In practice, players will find the existing classes effective when playing solo or in certain party setups. Jobs, however, will shine in party situations where specific skill sets are demanded.
If that's the case then that takes care of some of your concerns up there. It doesn't seem as complicated & redundant when I look at it that way.