Could the introduction of new classes change the Class/Job dynamic?

[Bixby]

Would access to 15 classes' abilities push jobs onto the back burner?

Only for a patch. If they stick with their current design intent, and people find a way to make classes superior to jobs, it'll be patched out.

If they change their stance and decide to make classes relevant, that's a whole other thing, but it'd take more than just a big selection of cross-class skills.