It's not that AST is trying to replace WHM or SCH. The problem stems from the fact that Diurnal is better than Nocturnal in a handful of ways and is better supported by the AST's kit. Here's why:

Diurnal - TD and CO can increase the remaining time on your buffs: your three HoTs (Asp Benefic/Helios and CU), your cards, and LA (by CO only but still). Your HoTs also stack with each other.

Nocturnal - You can also use TD and CO to extend all of the same abilities, but there is a significant drop in usefulness when you extend the duration of a shield that can break. These two abilities also don't stack.

Suggestions -
- Allow Aspected Benefic/Helios to stack.
- Provide some other benefit to Asp Benefic/Helios that stays even if the shield is broken. A Shield over Time on Asp Benefic/Helios
Numbers would need to be tweaked to keep AST balanced, but something like this could help Noct stand alongside Diurnal instead of in its shadow.