It got dumbed down way back in 1.0 when the ability change update happened and even further when the job update happened. Classes were originally envisioned as a way for people to customize and create their own unique class, and presumably certain combinations of abilities would create a job, of course with the game so badly rushed as it was it never really exactly came to that.

With all that gone and everything railroading you into a single set of abilities, there's not reason to keep classes anymore aside from the obvious coding aspects. It's too late to go back now to introduce more customization, so might as well remove the classes altogether since no endgame content requires them, when even in 1.0 you could get away with certain classes in endgame, mostly THM for a situational but useful AoE Sanguine Rite.