Each job is just a specialized version of the class. There's nothing wrong with a new job being released and you being Lv50, you just need to learn to use 5 skills that will tilt the class's specialization one way or the other.
You're thinking jobs as jobs. In this game jobs are just a way to play your class. No job changes any class fundamentally to the point it's unrecognizable. I don't see why they should be separate exp bars.