I think ASTs should be glad they still get to keep a bubble that mitigates AND applies a very potent regen. Under no circumstances does an AST need to stand there for 15 seconds in CU, so this argument of not being able to do anything is pretty irrelevant. The whole nature of Sacred Soil is to put it down for one heavy raid wide damaging move, as such an AST can do the same without wasting more than one GCD and they get a regen effect on top of that.

It's not going to be a game changing nerf, and ASTs are more than capable healers after their chain of buffs. There is no reason why they should still be getting special treatment on every move.