Your reasoning isnt in the good way. Every healer at the moment can heal at the same level. Sch bring fairy and substained dps with various buffs (and even yoshida said he was OP right before the last AST buff went live), AST now brings huge utility, but WHM is still the same.
So what people are asking is why take WHM, when you can have AST and SCH?

By your reasoning, we can remove all dps skill of every healer and every dps boosting skills, because it doesnt impact the raw healing output of the healers. That's completely wrong.