> Java lacks the expressiveness to make it much better than this.

That's not really true. You can have a Java library providing:

    Json.writeTo(System.out)
        .object()
            .array("providers")
                .element("SUN")
                .element("SunRsaSign")
                .element("SunEC")
            .end()
        .end();

or as a shortcut

    Json.writeTo(System.out)
        .object()
            .array("providers").withElements("SUN", "SunRsaSign", "SunEC")
        .end();

or similar (aka faceted fluent API), with typed interfaces mirroring the grammar of the target language, here JSON.

This is basically a structured output iterator (or OutputStream-like) pattern. It can also support modularization such that substructures can be factored out into separate methods or lambdas, thereby also allowing loops and conditionals.

Such an API can be provided in a relatively compact fashion and would be nicer than what the JEP proposes.