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I think it's hard because they have to rehaul their whole CORE system of things. To my mind and as an example:
Healers don't get challenged in healing. I barely use my kit during dungeons, there's no damage triage for me to make decisions on who to heal during pulls, even in savage it's very situational when I have to focus my healing on a particular person (P8S purple circle thing comes to mind).
Tanks don't get challenged in tanking, there's no aggro management in any of the normal content, and extreme and savage content have resorted to just tank swaps.
There's no gameplay difference between casters, melees and ranged. Something like a Ninja should be able to do things like SCOUTING, Ranged should be able to trap (MCH/BRD) or Charm (DNC) CC things, Casters should be able to nuke things (BLM, PCT, SMN), other than their rotations/buffs their role in any given content remains the same.