As a lawyer, I want to provide a legal perspective on your point.

The legal system has a lot of trouble dealing with decisions which have an obvious decision-maker who is also obviously unqualified to make the decision. In a child custody dispute, asking the child to decide is the intuitive answer. But there's no reason to think that a young child (i.e. 4 years old) will make a decision that won't be regretted later. In short, who lets a child play video games longer is not likely to be the heuristic that the adult version of the child would recommend. So the legal system has a standard heuristic trying to predict what a competent decision-maker would do. As you say, the fictional competent decision maker looks a lot like "not doing things with consequences that others don't want."

The reason I mention this is to state that judges are quite aware that the legal fiction is fiction. "Best interest of the person" is the name for this legal doctrine, not an assertion that the judge has figured out what is actually best. And the judge you quoted knows that. I understood the quote from the judge as recognizing that the fictional decision process should not be applied if the decision maker is actually competent to make the decision.