Your intuition about the benefits of pure functions & immutability are spot-on, and I think that any proponent of monads would agree that these are fundamentally more important concepts than monads themselves. (Many of the ways that people use monads would break, or lose their point, if the data they operated over could be mutated outside of the monadic context.)
You can get an awfully long way with immutable data structures without involving monads. There are plenty of languages that give priority to immutable data, but don't have monads in any inherent way: Prolog, Erlang, Ocaml, and Scheme (to a slightly lesser extent) all come to mind.
You can get an awfully long way with immutable data structures without involving monads. There are plenty of languages that give priority to immutable data, but don't have monads in any inherent way: Prolog, Erlang, Ocaml, and Scheme (to a slightly lesser extent) all come to mind.