The problem is this:
The more time healers are required to spend actually healing, the less time they have to spend patching up people's mistakes. That's the fundamental trade-off. Time is not a fungible quantity.
Now, perhaps that's a fair trade off in Savage and Ultimates. (I'm not qualified to comment there.) But normal mode content, anything that NA/EU queues up for in Duty Finder? It's utterly unrealistic to expect few to no mistakes / perfect play at that level. This means either
required healing cannot be tuned that high, or encounters need to be designed such that mistakes
cannot be made -- the latter reminding me of early Final Fantasies, where the trash and boss are on the left side of the screen, and the party is on the right side of the screen.
Edit: Which is not to say that the current balance of required healing is fine. It is not. I do believe there is room for increasing healing requirements, even at the normal/Duty Finder level, via various mechanisms: bringing back crit autos and untelegraphed mini-cleaves, reducing the self-sustain of other jobs and roles, rebalancing where a healer's healing potency actually comes from (GCD vs. oGCD), etc.