Is a Copula Subject a Predicate?

A true predicate will absolutely have the function of at least two. Some languages don't care for adjectives in copula complement position, either because their adjectives are actually verb-like or for other reasons they just don't like it. In some languages, I think Yoruba is one. You cannot say, you know, he is good. You have to say he is a good boy. Good is good is like a state of verb. So yeah, that works out a lot. And there can be a lot more interesting than most of us know, in my opinion.