The short version of it, AST is viable to replace SCH but not WHM, however SCH is still better.
The long version, AST gives piety as a party bonus stat, that alone is almost enough to not want it as a WHM replacement, if you also consider WHM aoe heals you just won't even think about replacing WHM.
Replacing the SCH is viable since you have more utility, but SCH is still far better since he can do more damage, more damage mitigation, help heal at the same time he dps (selene) and he also brings some utility (selene).