Yes but once the json has been validated once at runtime, you know it's safe for the rest of your program to use.

Whereas in a dynamic language (even an immutable one), your data could be mutated during program runtime and no longer conform to the type you expected. Yes you could re-validate it everywhere, but that requires you to exercise every code path when testing at runtime to find bugs.

I think the general argument that "You can't static type everything (like your API dependencies), so there's no point in doing it all" isn't a very solid one.

Your data can't be mutated if you're using immutable data structures. Which is what Clojure defaults to, in fact.

I didn't mean literally mutating the same structure in place - but a new version of your data structure could be returned from a function which changes some of the keys/values in unsafe ways.

Sure, my example isn't tied to static-typing but static-typing is especially helpful any time you're workin with combinators/transformations at a distance. But also, statically-typed languages like the two I brought up tend to be in most need of a nice validation/codec/transformation library because you can't simply procrastinate establishing your domain models like you can in dynamically-typed languages. And this is the sort of pain that leads people to say that dynamically-typed languages are more accommodating here.

Though I will have to defend Clojure from your last paragraph. I don't think there has been a lisp that has done so much for lisps than Clojure. For example, until I used Clojure, other lisps were just like any other dynamically-typed, mutation-heavy language. The idea of just shuttling around data and transformations on that data is something that fundamentally changed my approach to writing software.

I can certainly come up with worse things to say about Clojure. I kinda grew out of dynamic-typing, for one. And this is never more obvious than if you've ever read a large or complex Clojure codebase while dying to just know what the underlying Java types are. And you're so desperate that you get excited when you see a single ^ChannelHandler type-hint.