That's a neat argument, but it relies on "could" meaning "is willing to" more than (as I would argue) "is able to" in the sense of "is not being prevented from".Consider the case where someone is very concerned about an issue and is not willing to reduce his or her level of concern. Surely, that person "couldn't care less;" that person is unwilling to show less concern. The statement relies on interpretation to determine why one's level of concern cannot be reduced.