I can count on one hand the number of times I've had to do casting in Scala over the course of writing four different web applications of various sizes in it. Fine-grained control over side effects is only a benefit if you consider it a benefit, and is IMO far from the greatest benefit you can get from a strong, flexible type system. Easy mutability has little to do with type safety—indeed, type safety makes mutability marginally safer by at least holding you to a basic promise when you make mutations. I can't deny Scala is heavier than JavaScript, I suppose, for some definition of heavier that I assume means “has more features”, or means something that ends up equating to same.
To argue that writing software in both Clojure and Java (or pick 2+ other languages that all run on the JVM, of course) will leave you with an objectively lower complexity:benefit ratio than using a single language is a dubious claim indeed. I don't guarantee that just using Scala will always result in a lower such ratio, but you'd have to make a much better quantitative argument to convince me that you can have a lower ratio consistently by using different languages instead.
To deal in increasingly-divergent analogies one more time: if your work is in Las Vegas, you may not care that there are cheap supersonic flights to other cities. They will never compare to being able to do all of your work in Las Vegas, even if it means switching hotels depending on your needs on a given day.