Propose it to them? You'd get a lot of players coming back if other job classes are playable past 99.
I think it's a bad idea. This means they have to make an additional tier to those classes, which may potentially mess up many things...
For example gunslinger line becomes like this:
novice -> gunslinger -> expanded slinger
Now, when rebellion arrives, they would be hit with a dilemma: should they allow normal slingers to change job, or allow the expanded ones to change to rebellion as well? That means they may have to make a 4th job tier to accommodate for expanded slingers to job change, depending on their implementation.
Furthermore, what happens when you change to an expanded slinger? To break the limit of the level cap, you need to "change" jobs. Your job level gets reset. What happens to job bonuses and all? Will you get extra skill points and go till job 50? Or remain at job level 1 forever?
If job level remains at 1, how would job changing to rebellion from there work? The NPC needs to do extra checks, which are troublesome.
I like the idea of being able to train lower classes to higher levels, but implementation is a huge issue.