Since it requires runtime checking anyway, that code has to live somewhere. The decoders are that code. I don't understand how changing the runtime representation would allow to not have those decoders. Unless you mean they should be implicitly generated by the compiler. Or maybe I'm just missing what you're saying.
My point was about the sentence from the original post - its not the consequence of sound type system, its the consequence of choice of runtime representation. If runtime checking matters (which it does in practice), then we don't get any advantage in terms of simplicity by using "unsound" system, because we need to write that code anyway.
