As I've learned more and studied more math though, I've come to the conclusion that the relation really is the more fundamental primitive. Every function can be represented as a kind of restricted relation, but the converse is not true, at least not without adding considerable extra gadgetry.
While of course relational databases and SQL are the best known examples of relational programming, and are highly successful, I still believe it's a largely untapped space.
However, my interest currently is less in the space of programming language design and more in teaching very young children the basics of math. And for whatever reason it's considerably easier to teach a predicate like "is big" as a 1-ary relation and "is bigger than" as a 2-ary relation than trying to capture the same concepts as functions.