To add, the other roles can potentially take the healing load off the healer. RDM's flavor requires it in fact (though it also requires it to be limited), and SNM, PLD, DNC, DRK, SAM, GNB and WAR have Raise or potent (self-)healing options. Even melee have Second Wind and Bloodbath.

For shields Tanks obv have more of that, but BLM also has Manawall, NIN has Shadeshift, MNK has Riddle of Earth (nerfed as that is). and SNM has Titan's stoneskin.

All of this and Healers having attacks at all means that every role has little bit of the rest. Now this is by design for the Solo experience, but it also means that this self sufficiency opens out ways for the Healers to stave off having to heal as much as possible.

Which means that if the devs really want to make pure healers, they have to scrutinize their decisions with the quest design since ARR and the other roles. And that they would have to remove turn DPS into Glass Cannons, Remove damage form all Tank attacks (effectively making them the equivalent of showboating attention hogs) and obviously remove damage spells from the Healer kit. And from there change the quests (and Leves/FATEs) so that people can still clear them without having to deal damage for Tanks and Healers or survive the encounters as a DPS (prolly by adding NPC allies)