To emphasize, of course the monad interface isn't useful if the only implementation of it that you use is also the monadic implementation used by an imperative language. But if you "just" use an imperative language, you lose all the other implementations because the language hardwires the "conventional" one at the bottom. You lose the parser implementation of the monad interface, the free monad implementations (which are useful for writing "programs" that are then separated from their interpreter), the software transactional memory implementation, the probability implementation, the effects-controlling implementations (that go beyond "either in IO or not" to implement more granular controls), and the many other things that have been implemented.

Obviously that's ultimately a viable solution since almost every programming language works that way. But if you're going to ask "is the ability to implement the monad interface worth it for some given language?", you aren't going to get anywhere if you consider only one of the many implementations of the interface.

Why would anybody use the iterable interface if all it could do is the equivalent of "for (i = 0; i < arr.len; i++)" and couldn't iterate over a binary tree or hash table as well? Why would anyone use any interface if an interface only has one implementation, ever? The problem lies in the question, not the value of the interface.

(I'm trying to get better at phrasing my posts about "monads" better because one of the major problems they have is that people think of "monad" as a noun. But it is an interface, just like in Java or a Rust trait, that can be implemented by a type; it just so happens that the interface is not something that can be cleanly represented in most statically typed languages because of the high-order polymorphism. "Monad" is an adjective, not a noun. And "monads" aren't "about managing state" any more than the Iterable interface is about "incrementing an integer to walk over an array"; the conceptual error is the exact same kind in both cases.)