The problem is that if the current design meta of 'Toss large amounts of AoE with low enough movement and enough clustering that AoE healing will take care of it', together with 'toss mediocre amounts of individual/spot damage that only one healer has to be busy fulltime at most and another healer only has to throw super-efficient heals' makes SCH's strengths stand out tremendously as an offhealer compared to Nocturnal AST.

Aetherflow is already competitive with Shroud + Assize and Luminiferous + Celestial. Another thing often forgotten is that you start with 3 stacks too, and that extra mana can swing things pretty nicely.
Energy Drain is a huge boon if you don't need the healing. It is way harder to convert abilities like Collective, Assize, Asylum into mana-efficiency than it is to press a button and get MP back. And then we're ignoring the free damage that comes with it.
Even if you don't use Energy Drain, the abilities are ridiculously efficient anyway when compared to their spell-counterparts (think Succor/Adlo/Physick vs Indom/Lustrate).
Nocturnal AST has no answer to the fairy besides manually healing. Even if you slot in a Diurnal AST/WHM instead, you'd still need to spend mana on Aspected Benefic/Regen to replicate the effect (to a lesser degree at that) while the SCH is pushing tons of free healing while.. doing absolutely nothing. Absolutely nothing? Absolutely nothing.
SCH has more, usually competitive, on-demand abilities compared to Nocturnal AST that are very comfortable to use too, on top of abilities AST can't access or are locked behind RNG to produce a sub-par effect (AoE bole vs Fey Covenant as biggest offender, no answer to Deployment Tactics as examples).
Definitely has a lot more 'cleave' damage that is very cheap to use (Bane). Rub Aero 3 and Assize all you want, but Bane's absolutely crazy if you are allowed to let it tick.

I like Nocturnal AST, but it is very unrealistic to think WHM + N.AST will be a staple if Midas content is still based around pushing numbers, regardless of where. Giving up such a huge comfort and benefits while SE has already shown their designs usually don't change around (huge AoE damage/predictable tank buster/no-cast cleave(semi-buster)/low critical movement/high amounts of clustering) is pretty crazy.

If you want to see N.AST be really competitive, either pray Midas is a blow-out numbers-wise to the point the instant shield is a giant boon, or pray Midas will have a lot of spothealing/separation and very little clustering while tossing enough damage to keep both healers occupied. Or there are actually meaningful changes.