Before going on an impassioned speech defending this subject it would probably help to know TC rather specifically said he wants this for no real reason beyond making it easier to level up all of the jobs. They didn’t even meaningfully pair up jobs except for that purpose.

As for the rest, not happening. It throws off balance too much. They aren’t going to let you remove part of your core moveset, or in general be variable from another of your job. If you want that Blue Mage exists in part for that, and due to not fitting the game properly.