Currently, one of SCHs largest strengths is fight design. SCHs best healing and mitigation tools come from their cooldowns. With how scripted the fights are currently designed, it doesn't take too long to determine the optimal moments to use the correct cooldowns.

If fights were designed to be much more random in nature, it would put a monkey wrench in the SCH kit as it becomes more difficult, if not impossible, to have an ideal CD rotation for the SCH. That would push SCH away from being the optimal choice for those particular fights if this were to occur.

Upping overall healing frequency would also potentially put a damper on SCH because, again, their best tools are cooldowns. You'll notice that despite the strength of SCH, there are very very few SCH solo heals in the 3.X era versus the 2.X era. This just showcases how limited SCH can get.

The strength of the SCH kit is not just the kit as a whole, but also revolves around fight design and party design. If that gets shaken up in some capacity, you may see SCH slip out of the Meta as players shift to more favorable and direct healing that AST and WHM can provide.

Not to say and of the above will happen in 4.X, but it's one of the contributing factors to SCHs current position.