It isn't. It's also not an idea that anyone here has fought against -- only that the solution is not
simply to flip the table into 92+% of casts spent on healing, as that leaves almost nothing left for recovery, and spamming Medica is not particularly any more interesting than spamming Glare.
And by the time you get the portion of GCDs heals down to an amount still remotely accessible, such as a "mere" three-quarters of casts, the downtime kits of jobs like SCH would still feel flat (ED wouldn't be worth using and that leaves them with just Bio per 30s and Broils between).
That is to say, rather than just the one, it requires at least a
few steps:
- Increase healing requirements to, MP being mostly a non-factor, what might makes fights feel threatening (even if not yet interesting) for healers but short of outright wipefest hellscapes.
- Address the (im)balance of healing spells relative to each other through categorical (only this one player needs healing and/or they need it faster than Medica can produce) and/or shared-resource (e.g., MP, AF, AG) concerns or through just curtailing the most absurdly powerful spells among them. Ideally, take that opportunity, also, to make it so the spell kits aren't basically just the same basic copy-pasta across every healer. By the end of this, the healing itself should be pretty interesting.
- Provide a sufficiently engaging downtime kit to handle at least what flexible space remains in group content among skilled players. This may not be enough to solve healers being dull in solo-content, but solo content is not exactly the point of an MMO. Ideally, find ways for this kit to interact, even if indirectly or just coincidentally (such as through priority conflicts in spending refreshing two status effects, one offensive and one defensive), with the healing kit so that each is made more interesting through the other's presence. By the end of this, healing in group content should be thoroughly engaging (even if not solely do to the healing actions themselves).