Well in that case, there are a few things that needs to be addressed
First of all would be the requirement of the whole player base to stop tunneling into White Mage or Diurnal astrologian being the "main healer". As many perceive Nocturnal Astrologian as a healer similar to scholar, they tend to get treated as a scholar as well. Which isn't odd. Considering our lord and saviour Yoshi P made the comparison himself.* This is exactly the part where we need let go if we want to "break the meta". In a sense, a White Mage is more suitable to take the off-healer slot than Nocturnal Astrologian. Simply for their ability to keep healing while being in Cleric's Stance - Which off-healers are more in than not compared to the other healer with the meta. This is due to having Regen available. Which Nocturnal Astrologian does not, as it's equivalent is locked in Diurnal Sect. While either stance does have access to Collective Unconsciousness, it cannot be maintained indefinitely and white mage has something similar called Asylum. So as a pure napkin theory, Nocturnal Astrologian taking the main healing slot and White Mage taking the off-healer slot seems more promising than the other way around. I made a topic about this a few months ago too.**
However, and this brings us to the second point, stamina is a problem. White Mage simply does not have the ability to fulfill the role to primarily do damage while also doing their share of healing. Compared to Scholar the MP efficiency on damage spells is terrible (with exception of Broil) and their MP recovery ability doesn't scale with MP pool. There is also significantly more effort required to do so by switching back and forth between maiming and mending more frequently than a Scholar would. Even if it's only to re-apply Regen every 21 seconds. But, in my opinion, effort shouldn't be a primary issue here.
While I don't believe the meta would shift from astrologian+scholar being the most optimal duo if said issue would be addressed. I believe it does lessen the preference gap between the healing compositions available. No hard nerfs would be required. Maybe tuning shroud to be on par with Luminiferous Aether or lower the damage spells cost would be a good start. Or make said skills scale on MP pool like how Aetherflow does. As this is the only ability that only gets better as our gear progresses.
* https://www.bluegartr.com/threads/125199-Famitsu-PAX-East-Interview-(3-14)-Translation
** http://forum.square-enix.com/ffxiv/threads/279963-Astrologian-impossible-to-balance-as-it-is