An alternative to nerfing SCH is to also change the content design and make damage more randomized and spiky. It's unlikely to happen given S-E's perchance to mechanical dances, but if it does occur, the SCH kit will become indirectly less appealing.

Consider the fact the SCH kit is build completely around CD management. The last thing you want to do on SCH is use GCDs as an emergency heal tool because of how expensive their spells are compared to WHM or AST.

This kit style is completely synergistic with how content is designed - where players do the mechanical dance and have a healing answer to every tank and raid buster in the game. When SCH can plan out their CDs to a tee to every buster in the fight, it makes for an incredibly strong healer that has little weakness in that type of environment.

Outside of content design changing, I unfortunately feel the only way to see SCH fall out of its' meta spot is if it gets nerfed so hard that it basically becomes unplayable. The kit style works just too well with content design and any buffs to N.AST to try to usurp SCH's position has the adverse effect of also affecting D.AST more often than naught, making the gap between D.AST and WHM even further and just solidifying the AST/SCH composition even more.