This is and isn't true. There's nothing in ARR's design which suggests a hybrid of melee and magic, including the range differences, would be infeasible. The real problem is that Red Mage traditionally has healing spells in its repertoire. We currently have one healing spell on a job which otherwise does not heal (SMN with Physick), and that is from a class which also has a healing job (SCH). If you remove healing spells from Red Mage, then implementation is trivial, but it's also a rather odd Red Mage. If you focus on healing, it's simply not a Red Mage at all.
FF11's RDM wasn't a Red Mage, either. It was an abomination.