Then it stops working, and there is only one lever

Children habituate to noise the way everybody does. The volume that stopped a four year old does not stop the same child at six.

At that point there is exactly one lever available, which is more. Louder, or longer, or a harder edge to it. That is the escalator, and nobody steps onto it on purpose. You step onto it because the old level stopped producing the result, and the only adjustment the method allows is upwards.

This is worth knowing for one practical reason. If shouting is your main tool, the amount of it required goes up every year. Not because your child is getting worse. Because the tool wears out.