I mean, they made a mechanic where Macrocosmos was an "I win" button. I'd hate to think that no one thought to consider macrocosmos when designing P3S, but honestly seeing how little care gets put into healers in this game, it's almost assuredly what happened.
But in all fairness, those situations are totally fine. As long as each healer has different fights where they have their own shining moment, and it's not just one healer that's always the objectively right answer, then that makes healers more interesting and fun. You don't need Astrologian for P3S, but it's cool to see how great it is for that paricular fight. Having a single target that needs to be healed from 1 to 100% like you said would be great to make so that White Mage can actually be good at something for a change anyway. And that does add a lot of value to "Pure healers" since they are the two that have the least value as far as healing and mitigation are concerned.