I've always seen classes as more of a lore thing than a mechanical thing. Classes like lancer and conjurer are reasonably common in the world, but dragoons and white mages are the stuff of heroes. It makes sense for your novice level 1 character to start with a class and eventually promote. That the job overlaps the class rather than just overwriting it makes little practical difference... with only a couple rare exceptions you take your job at level 30 and stick with it forever.

The new jobs not having a class implies that they're advanced jobs that novice adventurers, as common folk, simply do not have access to. It doesn't need to be any more complicated than that. Though if they do overhaul the entire system I would be very curious to see how they go about it.